Involute profile and helix are the two elemental deviations for controlling the quality of gears, which are embodied in several gear accuracy standards. In this case, the main function of gear measuring instruments is designed to measure these two deviations. In fact, there is another significant characteristic curve beside the profile and helix, that is, the profile of the path of contact. During the gear hobbing, shaving, grinding and other generating machining, this curve is consistent with the machining curve on the tooth surface. During the involute helical gear transmission, this curve is also consistent with the working curve on the tooth surface. Therefore, the quality of machining and transmission of the gear can be reflected by the profile of the path of contact. However, the existing gear measuring instruments cannot measure it. In this paper, combining the forming principle of the profile of the path of contact，its theoretical model is given. Based on the existing gear measuring center, two methods for measuring the deviation of the profile of the path of contact are proposed, including the four-axis method and the three-axis method. In terms of measurement methods, the measurement of the profile of the path of contact and the involute profile is unified. The measurement practices show that the measurement and evaluation of the profile of the path of contact can be conveniently realized by the existing gear measuring instruments. As the actual working curve on the tooth surface, the profile of the path of contact is unique. In essence, it is the combined result of profile and helix, which can characterize the quality of gears. It can not only be employed for analyzing gear process errors, but also for predicting the dynamic performance of gears.