The influences of linear and nonlinear phase noise on the performance of single channel 40 Gbit/s RZ-DPSK transmission system were analyzed in detail. The result showed that the linear phase noise induced by ASE noise was of the most significant influence, the nonlinear phase shift induced by self phase modulation took the second place ,and IFWM had the least influence. With optimum input power, the differential phase Q-factor penalty was more than 204 dB for ASE-induced linear phase noise and less than 7.5 dB for G-M phase shift. The result showed that the ASE-induced linear phase noise was the major limmitation to DPSK transmission systems with lumped EDFA amplifiers.