可以使用UIBezierPath的

• (UIBezierPath *)bezierPathWithRoundedRect:(CGRect)rect
  byRoundingCorners:(UIRectCorner)corners
  cornerRadii:(CGSize)cornerRadii

参数corners指定了想要需要成为圆角的角。可选值为：
enum {
UIRectCornerTopLeft = 1 << 0,
UIRectCornerTopRight = 1 << 1,
UIRectCornerBottomLeft = 1 << 2,
UIRectCornerBottomRight = 1 << 3,
UIRectCornerAllCorners = ~0
};
typedef NSUInteger UIRectCorner;
从名字就能看出来其代表的意义。
例子：
UIView *view2 = [[UIView alloc] initWithFrame:CGRectMake(120, 10, 80, 80)];
view2.backgroundColor = [UIColor redColor];
[self.view addSubview:view2];

UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:view2.bounds byRoundingCorners:UIRectCornerBottomLeft | UIRectCornerBottomRight cornerRadii:CGSizeMake(10, 10)];
CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
maskLayer.frame = view2.bounds;
maskLayer.path = maskPath.CGPath;
view2.layer.mask = maskLayer;
如果需要将UIView的4个角全部都为圆角，做法相当简单，只需设置其Layer的cornerRadius属性即可。