Cycloidal gear drives have advantages of compact sizes, large reduction ratios, and low backlash. They are particularly suitable for applications in precise positioning and large output torque, such as industrial robots and machine tools. Up to now, the effective precision evaluation of cycloidal gears is lacked in commercial measuring devices. The main goal of this paper is to propose precision evaluation for cycloidal gears. The research focuses on (1) establishing a mathematical model of cycloidal gear and (2) developing a method for precision evaluation of cycloidal gear. The evaluation items cover the following areas of tooth profile and thickness errors, pitch errors, accumulation pitch errors and runout. The profile points measured by a coordinate measuring machine are divided into tooth groups and then formulated as curves using a B-spline curve fitting technique. The precision of cycloidal gear can be further evaluated. A case study on Nabtesco RV-25N is displayed to verified the correctness of the proposed method.