As the basic part of the mechanical transmission, gears should have high transmission accuracy, so the gear's precision measurement is a key technology. Based on the existing research, the computer vision technology is introduced in gear test. According to the definitions of technical indexes, such as pitch deviation, tooth thickness deviation, etc., through analysis and research, a new measurement method is proposed, so as to complete the determination of the modulus, tooth number and total accumulated pitch deviation of a gear. In the proposed methods, those indexes such as the pitch and tooth thickness, are measured in arc length, rather than in chordal length, so that they are consistent with their definitions. And the measured points can be positioned on the reference circle of a gear while the pitch and total accumulative pitch deviations are examined. Finally, a gear is measured with the proposed method. The main test is obtained as follows. The single pitch deviation is 10,3 mu m, total cumulative pitch deviation is 44,8 mu m, and tooth thick deviation is 5,2 mu m. The experiment demonstrates that the proposed method is feasible and effective, and can meet the precision need of engineering practice. This study provides a new solution method for the gear's on-line measurement.