diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e138c73
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+SnakeGame.iml
+.idea/
+/out/
diff --git a/README.md b/README.md
index 1d35272..f58e588 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,4 @@
 # SnakeGame
-This is a snake Game writing in Java using JetBrains IntelliJ Idea
+This is a snake Game writing in Java using JetBrains IntelliJ Idea.
+
+Warning: history.txt is to record the score history of the game based on local computer. Delete this file will cause history lost.
diff --git a/Snake/src/GamePanel.java b/Snake/src/GamePanel.java
index 70b370a..4eda895 100644
--- a/Snake/src/GamePanel.java
+++ b/Snake/src/GamePanel.java
@@ -9,7 +9,7 @@ public class GamePanel extends JPanel implements ActionListener {
     static final int SCREEN_WIDTH = 600;
     static final int SCREEN_HEIGHT = 600;
     static final int UNIT_SIZE = 25;
-    static final int GAME_UNITS = (SCREEN_WIDTH * SCREEN_HEIGHT) / UNIT_SIZE;
+    static final int GAME_UNITS = (SCREEN_WIDTH * SCREEN_HEIGHT) / (UNIT_SIZE * UNIT_SIZE);
     static final int DELAY = 75;
     final int x[] = new int[GAME_UNITS];
     final int y[] = new int[GAME_UNITS];
diff --git a/Snake/src/Ranking.java b/Snake/src/Ranking.java
new file mode 100644
index 0000000..c3c683f
--- /dev/null
+++ b/Snake/src/Ranking.java
@@ -0,0 +1,117 @@
+import java.io.*;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * @author SoongMingLiang (soongmingliang647@gmail.com)
+ * @version 1.0.0
+ *
+ * This class is to provide the ranking system service to the SnakeGame.
+ * The methods that are available to other classes currently are
+ * getHistory(), insertScore(int new_score), and resetHistory()
+ * The main functionalities of these methods are return the game score history, insert a new score into the file, and
+ * reset the score history of the game respectively.
+ */
+public class Ranking {
+    private static final File historyScoreFile = new File ("history.txt");
+    private static ArrayList<String> scoreHistory = new ArrayList<>();
+    private static ArrayList<Integer> scoreHistory_write = new ArrayList<>();
+
+    private static void readHistory(){
+        try(BufferedReader scoreReader = new BufferedReader(new FileReader(historyScoreFile))){
+            String score = scoreReader.readLine();
+            if(score == null){
+                scoreHistory.add("0");
+            }
+            else{
+                while (score != null) {
+                    scoreHistory.add(score);
+                    score = scoreReader.readLine();
+                }
+            }
+        }
+        catch(FileNotFoundException e){
+            System.out.println("Caution!!!");
+            System.out.println("File is not found! A new file will be generated for you.");
+            System.out.println("Note: All local history will be clear out!");
+            try{
+                if(historyScoreFile.createNewFile()){
+                    for(int i = 0; i < 3; i++){
+                        System.out.print(".");
+                        TimeUnit.SECONDS.sleep(1);
+                    }
+                    System.out.println("\nYour history score file has been created successfully!");
+                    System.out.println("You are good to go now, kindly restart this program.");
+                }
+            }
+            catch(IOException | InterruptedException ioe){
+                ioe.printStackTrace();
+            }
+        }
+        catch (IOException ioe){
+            ioe.printStackTrace();
+        }
+    }
+
+    private static void sortHistory(){
+        Collections.fill(scoreHistory_write, 0);
+        for(var i : scoreHistory){
+            scoreHistory_write.add(Integer.parseInt(i));
+        }
+        Collections.sort(scoreHistory_write);
+    }
+
+    public static ArrayList<String> getHistory(){
+        readHistory();
+        return scoreHistory;
+    }
+
+    public static void insertScore(int new_score){
+        String newScore = String.valueOf(new_score);
+        scoreHistory.add(newScore);
+        sortHistory();
+        try(FileWriter scoreWriter = new FileWriter(historyScoreFile, false)){
+            for(var i : scoreHistory_write){
+                if(i == 0){
+                    continue;
+                }
+                else{
+                    scoreWriter.write(i + "\n");
+                }
+            }
+        }
+        catch (IOException e){
+            e.printStackTrace();
+        }
+    }
+
+    public static void resetHistory(){
+        try(FileWriter scoreWriter = new FileWriter(historyScoreFile, false)){
+            scoreWriter.write("");
+        }
+        catch (IOException e){
+            e.printStackTrace();
+        }
+        Collections.fill(scoreHistory, null);
+        Collections.fill(scoreHistory_write, 0);
+    }
+
+    public static void main(String args[]){
+        /*ArrayList<String> history = getHistory();
+        for(String s : history){
+            System.out.println(s);
+        }*/
+
+        /*insertScore(10);
+        insertScore(300);
+        insertScore(100);*/
+
+        /*ArrayList<String> history = getHistory();
+        for(String s : history){
+            System.out.println(s);
+        }*/
+
+        /*resetHistory();*/
+    }
+}
\ No newline at end of file
diff --git a/history.txt b/history.txt
new file mode 100644
index 0000000..e69de29