According to the proposal for ISO 1328 on the basis of DIN, the evaluation of tooth profile error requires the “mean profile” which is a modified version of the design profile. However, the determination of a mean profile has some troublesome problems. A simple and correct method to determine it by the method of least squares is presented. The determination of a mean profile can be carried out through the following three steps: (1) The least-squares fitted line of the design profile is obtained by the method of least squares. (2) The least-squares fitted line of the actual (measured) profile is also obtained. (3) The mean profile is provided as the sum of the following two components. The one is the slope component given as the least-squares fitted line of the actual profile. The other is the form component which is obtained by subtructing the least-squares fitted line of the design profile from the design profile itself. A prototype system, which consists of a personal computer and a digitizer, developed due to the above mentioned principle.