Abstract: A pair of mashed gear sfixed in an external spur gear pump is regarded as the heart of the oil pump. Whether gear parameters are selected rationally or not affects its performance, noise and working ilfe directly. In the past twenty years or so, the method of calculating gear paramters, which was first adopted by Porf. Jokin of the Soviet Union and prof. Of Japan, has been widely used both at home and abroad. Showing a series for analyses and calculation, this article gives some evidence to indicate professors that they have paid slight attention to the fact that the addendum factor exerts a tremendous influence on the functions of the gear pump. Both analyses and practice have proved it important to select gear parameters rationally. They can help us to make the oil pump smaller in size, the bearing carry less load, and the rate of flow pulse decreased on a larger scale. A new viewpoint is put forward in this article, that is, if the number of gear teeth is the same, the magnitude of the displacement of the unit volume is the only factor which determines whether the gear parameters of the oil pump are selected rationlly. The greater the value, the more rational the gear parameters.