A popular approach to optimize the performance of a gear transmission system is the modification of the gear tooth microgeometry, which includes the intentional removal of material from the gear teeth flanks, so that the shape is no longer a perfect involute. Such modifications compensate teeth deflections under load, and the resulting transmission error is minimized under a specific torque. Therefore, micro-geometry modifications can be applied on the involute (or profile) and lead of the gear teeth. In order to study the tooth micro-geometry optimization, this research selected a mass produced planetary gear type drive reducer as a prototype. The original design was modeled by a commercial software named Romax Designer, which analyzes and optimizes different types of gear power transmission systems. The series analysis results obtained reveal some anomalies that require modification. Based on the result, optimization and gear tooth modification were done to deal with the load distributions on gear tooth, gear durability problem, safety factor and bearing life problem. This thesis presents the outstanding performance improvement obtained after such optimization.