This paper reveals the internal relations between the amount of actual oil supply and the constant variable oil film thickness on tooth faces according to the characteristics of gear drive . It explains and analyzes the mechanism of the influence of oil supply on the lubrication effect and the cooling effect of tooth faces according to the basic theory of modern science and technology system. Therefore the research and application to the oil supply theory have been brought within the modern scientific field and the scientific basis for theory has been proveided for the analysis and reveiw of the practicable use of all the different views of oil supply and for the selection of the optimum oil supply scheme for the gear.