Abstract: Underwater wireless optical communication (UWOC) systems have higher bandwidth, lower attention, lower time latency, lower power loss, better security and higher communication rate compared to acoustic communication as well as underwater radio frequency communication. UWOC systems receive growing attention as an alternative technology to meet the high-speed and large-data requirement in various underwater applications such as underwater broadcast data, imaging, high-throughput sensor networks, even the real-time video transmission. Despite all these advantages, expect absorption and scattering effects, the turbulence effect under water, will also cause loss and fading on the received optical signal, and limit the viable communication range of UWOC systems. The improvement of differential phase shift keying modulation (DPSK) to the bit error rate (BER) of UWOC system is presented. Formulations of plane and spherical waves propagating through strong oceanic turbulence, which described as Gamma-Gamma model, are obtained by using the modified Rytov method. Based on these formulations and the characteristics of Whittaker M function, the analytical expressions for BER of a DPSK modulated UWOC system are derived. The analytical expressions are verified by comparing with the numerical calculations of the definition. The system performance is simulated under three important oceanic parameters and the propagation distance L. Also the system performance of the DPSK UWOC system with the on-off keying (OOK) UWOC system is compared. The simulated results show that the performance improves when the UWOC system adopting spherical wave with DPSK modulation at a lower value of the rate of dissipation of mean-squared temperature, the ratio of temperature to salinity contributions to the refractive index spectrum, the propagation distance, or a higher value of the rate of dissipation of kinetic energy per unit mass of fluid.