In order to increase the amount of low viscosity index bright stock used in the gear oil, the viscosity index of bright stock was improved using three different kinds of viscosity index improver, and the effect of viscosity index improver on anti oxidation, anti emulsification, anti foaming and tribological performance of the base oil was evaluated. The results show that Ethylene alpha olefin oligomer has excellent ability of improving viscosity index, shear stability as well as little influence to anti oxidation capability of base oil. Ethylene propylene copolymer has most poor shear stability. Polymethacrylate obviously decreases anti oxidation as well as improves extreme pressure and antiwear performance of base oil. 