A gearbox is often used in a wind turbine to increase the rotational speed from a low-speed main shaft to a high-speed shaft connecting with an electrical generator. Gears in wind turbine gearbox are subjected to severe cyclic loading due to variable wind loads that are stochastic in nature. Thus, the failure rate of gearbox system is reported to be relatively higher than the other wind turbine components. It is known in wind energy industry that improving reliability of gearbox designs is one of the key points to reduce wind turbine downtime and to make wind energy competitive as compared to fossil fuels. However, a wind turbine is a complex multi-physics system involving random wind loads, rotor blade aerodynamics, gear dynamics, electrical generator, and control systems. How to get an accurate prediction of the gearbox lifetime is a challenging issue. Furthermore, although some studies about wind turbine gear failure modes are carried out, limited studies have been carried out regarding design optimization including the reliability-based design optimization (RBDO) of the gear system considering wind load and manufacturing uncertainties.