i use itouch 4G has my device and i use simulator-4.2 i will just draw a rectangle as an example. i use Quartz-2d to draw - (void)drawRect:(CGRect)rect { // Get a graphics context, saving its state CGContextRef context = UIGraphicsGetCurrentContext(); CGContextSaveGState(context); // Reset the transformation CGAffineTransform t0 = CGContextGetCTM(context); t0 = CGAffineTransformInvert(t0); CGContextConcatCTM(context,t0); // Draw a green rectangle CGContextBeginPath(context); CGContextSetRGBFillColor(context, 0,1,0,1); CGContextAddRect(context, CGRectMake(0,0,320,480)); CGContextClosePath(context); CGContextDrawPath(context,kCGPathFill); CGContextRestoreGState(context); } i run it in the simulator, the whole screen becomes green, then i run this on my device, only the quarter of the screen becomes green, in order to make the whole screen green on my device, i have to draw a larger rectangle CGContextAddRect(context, CGRectMake(0,0,640,960)); seem like my device has twice resolution than the simulator, how can i fix this?