Differential phase-shift keying (DPSK) format gained more attention for long-haul optical fiber communication systems, due to its advantages of improvement in receiver sensitivity, high spectral efficiency and robustness to nonlinear effects. Phase noise and nonlinear phase noise are the main factors impacting the performance of DPSK system. Phase noise is monitored and reduced by four-wave mixing (FWM) in a fiber experimentally. The results show that the products of FWM can monitor the value of phase noise, DPSK signals performance can be improved by a amplitude limiter in a fiber based on saturation FWM through reducing nonlinear phase noise. This result is very important for high-speed all optical communication.