angles in round corner div

When creating a round corner div with background image in the body, the corners have sharp angles. During a period of bewildered, i found in .css file the same color value of the sharp angles. That's because of the div has been set a 'background-color' with white color, so it will make the angles occured. After set the 'backgound-color' as 'transparent', the angles will disappear.