Under Gamma-Gamma atmospheric turbulence, the influence of atmospheric scintillation, channel loss and pointing error on the performance of differential phase shift keying (DPSK) coherent optical communication system such as the bit error rate (BER), channel capacity and outage probability is studied in detail. Free space optical communication (FSO) system is based on the Mach- Zehnder modulator (MZM) and Mach- Zehnder interferometer (MZI) DPSK modulation/demodulation structure. The closed- form expression of average BER, average channel capacity and interruption probability is derived by the generalized hypergeometric method and the simplified analysis. The simulation is taken place under the Matlab platform. Numerical results show that the average BER and channel capacity are decreased with the atmospheric turbulence increased, the channel capacity can be promoted by increased the bandwidth or reduced the jitter variance, and the communication performance of the system is seriously affected by the increasing outage probability owing to the overlarge pointing error.