In most current investigations on bearing forces in screw compressors, the supports at the bearings are basically treated as ideal, simply supported boundary conditions [Adams and Soedel, 1995; Qin and Adams, 1996; You et al, 1994]. By using static mechanics, the loads on the rotors are resolved to equivalent loads at the bearings at suction and discharge ends. Studies on rotor dynamics have shown that the characteristics of a rotor bearing system are strongly dependent on thebearings that support the rotor. Therefore, it is important to study the rotor dynamic performance of screw compressors so that the bearing faces can be more accurately described. In this paper, a dynamic model of a rigid compressor rotor supported by two cylindrical roller bearingsand a four point contact ball bearing is developed from basic principles. This model is used to simulate the dynamic response of a typical screw compressor configuration. Under compression loads, the resulting bearing forces in the screw compressor are compared with those obtained by assuming ideal simply supported boundary conditions at the bearings. A more accurate model of the compressor response is believed to be obtained by coupling the global rotor motion with the local dynamics of nonlinear bearings.