Multiple input multiple output (MIMO) technology using phase shift keying orthogonal frequency division multiplexing modulation (PSK-OFDM) in free-space optical (FSO) communication is studied and MIMO-OFDM communication system based on Gamma-Gamma atmospheric turbulence channel is analyzed. A closed-form expression of bit error rate (BER) for FSO communication is derived with Meijer G function. The simulation result shows that MIMO technology which uses PSK-OFDM modulation can reduce BER effectively over different atmospheric turbulence. As the turbulence intensity becomes stronger, the effect of MIMO technology in reducing BER is gradually enhanced.