Based on Gamma-Gamma atmospheric turbulence channel,the bit error rate (BER) performance of wireless optical communication system with pulse position modulation (PPM) is an alyzed.The closed-form expression of average BER is derived theoretically.The formula is suit able for a wide range of turbulence conditions and any modulation level.Compared with the numerical integral,it can increase the speed of calculation.For a given average BER,the comparative analysis of the relationship between the modulation level and the re quired average transmitted optical power is done over different turbulence conditions.The results show that under the n on-turbulence and weak turbulence channel,PPM yields an average power requirement that decreases with modulation l evel increasing.However,with the enhancement of turbulence,the effect of saving transmitted optical power become s weak with the increasing of modulation level.In the strong turbulent regime,increasing modulation level ev en leads to increasing average transmitted optical power requirements.In actual system design,possible turbulent environm ent should be considered,and then the right modulation level is selected.