A numerical simulation was performed to investigate the performance of oil-injected twin screw air compressor with the thermodynamic process of compression between the oil and air phases. A generalized approach method for numerical simulation was applied which implies the computation of screw compressor thermodynamic process. Numerical analyses were performed using a three-dimensional model with SST turbulent and implemented by commercial software, ANSYS Fluent. The numerical results of volumetric efficiency and discharge pressure were graphically depicted. In addition, the above results were compared with existing published CFD results and showed good agreement.