The effect of the atmospheric turbulence can deteriorate the performance of the free-space optical (FSO) communication system. A technique for using orthogonal frequency division multiplexing (OFDM) modulation in FSO is studied, the bit error rate (BER) performance of FSO-OFDM modulation system over Gamma-Gamma atmospheric turbulence is analyzed. Under different atmospheric turbulence intensity, the BER of OFDM modulation system is compared with on/off keying (OOK) modulation system. The simulation results show that OOK modulation system and OFDM modulation system are deteriorating with the strengthening of turbulence intensity, under different atmospheric turbulence intensity, the BER performance of 4QAM-OFDM modulation system is better than 16QAM-OFDM modulation system and OOK modulation system.