Due to the recent 4^th industrial revolution and explosive interest in smart factories, the demand for industrial robots and related technologies is rapidly increasing. In order to develop a precision reducer for robots, a measurement technology of the small teeth of a gear is very important. This paper developed a method to measure the tooth shape of a precision harmonic reducer for robots and to evaluate the tooth shape measurement error quantitatively. The tooth shape of the flexspline of a harmonic drive was measured using three instrumental devices: a stylus, a projector, and a laser line scanner. A mathematical tool was also developed to evaluate the error by comparing the designed and measured teeth. The measured tooth profile was quantitatively assessed with maximum, average, and root mean square error. The measurement method using a stylus has disadvantages of careful measurement effort and a small measuring range. However, it has better precision. On the other hand, the projection method shows a moderate performance without much effort during measurement.