Gears have been used as primary machine elements that comprise a power transmission system in many mechanical engineering fields. This paper presents research on the optimization of gear geometries. Drive unit consists of many spur gears in inner structure, and efficient alignment will be needed for fine performance of the unit. If mesh misalignments occur, load distribution would be unbalance and concentrated in one side. It is directly connected many problems such as banding deformation to shaft, gear and housing. As much as deformation occur these machine element’s duty cycle will be decreased. Moreover due to unbalanced load distribution, noise and transmission error cause serious defect on gear performance. In order to solve this matter, it is necessary to modify tooth profile for balanced load distribution. There are two different solutions. One is micro-geometry optimization and the other is macro-geometry optimization. In this paper we choose the first one because it just takes minimum gear modification but more effective. To implement this, the latest simulation technology will be used and it allows not only the defection of gear mesh misalignments but also how to modify gear profile and lead slope. Before make prototype gear production, it provides more helpful information.