The oil flooded screw compressor rotates with contact between the corresponding male and female rotors. As gas pressure rises in the grooves of the rotor, a periodically fluctuating torque occurs. An impact vibration between the male and female rotors is induced by the torque. In this report, impact vibration analysis is carried out considering movement of both rotors. The stable region of the periodical impact vibration, which causes a noise of compressor, is calculated. Except this region, the impact damped vibration only occurs, where the compressor is driven quietly.