self.navigationController.navigationBar.shadowImage = [[UIImage alloc] init];
或者
[self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc] init] forBarMetrics:UIBarMetricsDefault];
self.navigationController.navigationBar.shadowImage = [[UIImage alloc] init];
即可
原文地址:http://blog.csdn.net/darongzi1314/article/details/42458845