How to create a round button?

I want to create a round circular button. This button should look like a circle.

This code gives round rectangular button.

UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
button.frame     = CGRectMake(100, 100, 100, 30);
UIImage  *image  = [UIImage imageNamed:@"01.png"];
[button setImage:image forState:UIControlStateNormal];
[image release];

I figured out how to create a rounded rectangular button but I want to create a round circle button. What do I need to change?