In various machinery field, gear has been used to mechanical element for power transmission and control. In addition, the speed can be adjusted through the gear ratio. However when gear mesh misalignments occur, the load distribution of gear face width will be concentrated in one side. And it could cause bending deformation to shaft, housing as well as gear. This matter directly connected to noise and gear lifespan problem. To prevent this, it is necessary to balance the load distribution by modifying profile of the gear. As the latest simulation technology has been improved, it is enable to optimize profile of gear prior to prototypes. Thus, the simulation allows the detection of gear mesh misalignments and how to modify gear lead slope and profile slope.