Potassium dihydrogen phosphate (KDP) crystal, widely used for important electro-optic parts, is a typical hard-to-machine material because of its soft, brittle, and anisotropic properties. High quality is usually required for machined surfaces on KDP parts. Reported machining methods for KDP crystal include diamond turning, grinding, magnetorheological finishing, and polishing. Each of these methods has its limitations. Therefore, it is desirable to develop new machining methods for KDP crystal. This paper presents an experimental investigation on surface roughness in rotary ultrasonic machining (RUM) of KDP. It was found that the surface roughness obtained when using a tool with a chamfered corner was lower than that obtained using tools with right-angle corners. Other process variables (spindle speed, feedrate, and ultrasonic power) also affected the surface roughness obtained.