Abstract: The impact of Gamma Gamma strong oceanic turbulence and pointing error on the average bit error rate(BER) and outage probability of a heterodyne differential phase-shift keying(DPSK) underwater wireless optical communication(UWOC) system with an aperture receiver was investigated. The optical intensity fluctuation due to the combined effects of oceanic turbulence and pointing error was derived. The close-form expressions for the average BER and outage probability were derived. Then the average BER performance and the outage probability performance versus signal to noise ratio(SNR) of the considered UWOC system were investigated with different point errors, source beam widths, receiver aperture sizes and oceanic turbulence parameters. The results indicate that the larger the aiming error is, the worse the system performance is, under the same beam width and channel environment. Choosing a larger radio of source beam width to aperture radius or a bigger aperture receiver can help to improve the system performance. In addition, the system shows a better performance over the strong oceanic turbulence with a smaller ratio of temperature to salinity contributions to the refractive index spectrum ω and the rate of dissipation of mean-squared temperature χ_T or a larger rate of dissipation of kinetic energy per unit mass of fluid ε and the kinetic viscosity u. This work will provide reference for the construction and performance estimation of UWOC system on strong oceanic turbulence when taking pointing error into consideration.