This paper investigates the symbol error rate(SER) performance of radio over free space optics(RoFSO) systems under atmospheric turbulence channel.An average SER closed form expression of BPSK modulation format in RoFSO is derived under weak to strong turbulence condition.Based on the derived expression,the numerical simulation is performed.It is shown that the Rytov variance,the diameter of the receiver collecting lens aperture,and the propagation distance are the key factors which have influence on the average SER.The effects of these three factors are discussed.Finally,the SER performance of MPSK/MQAM modulation format in RoFSO system is studied.The best modulation format with high bit speed and relatively low SER is found.