The online monitoring to lubricating oil viscosity is an important way to achieve the mechanical condition monitoring and fault diagnosis,but in practical engineering applications,the realtime oil viscosity needs to be converted into the standard viscosity at 100 ℃ according to the oil viscositytemperature characteristics.To study the viscositytemperature characteristics of bulldozer gearbox lubricating oil of 15W40 CF4,the gearbox lubricating oil of SD22 bulldozer was sampled,and the kinematic viscosity of the new oil,oil in midlife and used oil was measures at the temperature range from 40 ℃ to 100 ℃.Based on Andrade equation，Walther equation and Vogel equation，regression analysis was performed to fit the experimental discrete viscositytemperature data of the oils.The fitting precision of these three kinds of viscositytemperature relations was compared.The results show that the higher accuracy is achieved when using Vogel equation．This method can provides the basis for viscosity conversion of realtime online monitoring of lubricating oil.