It is quite hard to tell so I upload an image to show my problem: http://i42.tinypic.com/2eezamo.jpg Basically in drawRect, I will draw the line from touchesMoved as finger touches and I will call "needsDisplayInRect" for redraw. But I found that the first line is done, the second line will clear the rect part, so some previouse drawing is gone. Here is my implementation: enter code here -(void) drawRect:(CGRect)rect{ //[super drawRect: rect]; CGContextRef context = UIGraphicsGetCurrentContext(); [self drawSquiggle:squiggle at:rect inContext:context]; } - (void)drawSquiggle:(Squiggle *)squiggle at:(CGRect) rect inContext:(CGContextRef)context { CGContextSetBlendMode(context, kCGBlendModeMultiply); UIColor *squiggleColor = squiggle.strokeColor; // get squiggle's color CGColorRef colorRef = [squiggleColor CGColor]; // get the CGColor CGContextSetStrokeColorWithColor(context, colorRef); NSMutableArray *points = [squiggle points]; // get points from squiggle // retrieve the NSValue object and store the value in firstPoint CGPoint firstPoint; // declare a CGPoint [[points objectAtIndex:0] getValue:&firstPoint]; // move to the point CGContextMoveToPoint(context, firstPoint.x, firstPoint.y); // draw a line from each point to the next in order for (int i = 1; i < [points count]; i++) { NSValue *value = [points objectAtIndex:i]; // get the next value CGPoint point; // declare a new point [value getValue:&point]; // store the value in point // draw a line to the new point CGContextAddLineToPoint(context, point.x, point.y); } // end for CGContextStrokePath(context); }