Abstract: The design of high-performance screw vacuum pumps is based on the accurate prediction model of the leakage process in dry screw vacuum pumps. In this paper,we summarized the models describing the Poiseuille and Couette flow in screw vacuum pumps' leakage channels based on literature research,which was suitable for the simulation of molecular,viscous and transitional flow regimes of gas. Thermodynamic simulation procedure was developed based on the above models to predict the leakage process in screw vacuum pumps. The leakage process was analyzed in detail for a fixed-pitch screw vacuum pump under the condition of ultimate vacuum.Resultsshowed that the Poiseuille flow at the rotors' tooth top was always the main factor affecting the internal leakage process in the screw vacuum pump under different rotor clearances and rotating speeds. The leakage at the rotor cusp had a great influence on the external leakage process. The leakage caused by Couette flow in tooth top and tooth meshing leakage channels should not be neglected. The main gas leakage route in screw vacuum pumps was summarized as follows:the gas in the high-pressure working chamber first leaked into the low-pressure working chamber through the tooth top of rotors,and then leaked into the suction chamber through the rotor cusp region.