I found a useful pieces of code that can let you request user to review your app. i would like to share it here.
Code:
NSString *str = @"itms-apps://ax.itunes.apple.com/WebObjects/MZStore.woa";
str = [NSString stringWithFormat:@"%@/wa/viewContentsUserReviews?", str];
str = [NSString stringWithFormat:@"%@type=Purple+Software&id=", str];
str = [NSString stringWithFormat:@"%@493860607", str];
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:str]];