In these days, there are increasing demands for high precision and high efficiency machining in the industry of optical parts. The purpose of this research is the speed-up of shaping with a single crystal diamond tool. In order to investigate the effect of cutting speed on cutting errors, a cutting experiment is conducted under several cutting speeds. As a result, periodic cutting errors appear on workpiece surface in cutting speeds above 10m/min. The pitch of the error does not depend on the cutting speed. In order to investigate the cause of cutting errors, the tool motion is compared with the workpiece profile. The pitch of the cutting error corresponds to that of a periodic component in the tool motion. The pitch of the cutting error increases when a lubricant is used.