In this paper a new mathematical model for calculating the indicator diagram of twin screw expander is presented. The geometric parameters related to the rotation angle of male rotor are used in the model such as groove volume, suction and discharge port area, leakage area etc. The effects of internal leakage through five paths, oil injection, gas-oil heat transfer and refrigerant property are taken into account simultaneously and separately in the theoretical analyses. To verify the model and the calculated ?-p indicator diagram, experimental recording of ?-p indicator diagram of twin screw expander was carried out. The results of theoretical calculation were in good agreement with the experimental data, which lead to conclusion that the model could be used as a powerful tool for performance prediction and product development.