Nowadays, lower gear vibration and noise are necessary for drivers in automotive gearbox, which means that transmission gearbox should be optimized to avoid noise annoyance and fatigue before quantity production. Transmission error (T.E.) is the excitation factor that affects the noise level known as gear whine, and is also the dominant source of noise in the gear transmission system. In this paper, the research background, the definition of T.E. and gear micro-modification were firstly presented, and then different transmission errors of loaded torques for the spur gear pair were studied and compared by a commercial software. It was determined that the optimum gear micro-modification could be applied to optimize the transmission error of the loaded gear pair. In the future, a transmission test rig which is introduced in this paper is about to be used to study the T.E. after gear micro-geometry modification. And finally, the optimized modification can be verified by B&K testing equipment in the semi-anechoic room later.