In atmospheric channel, atmospheric turbulence is one of the main factors that degrade the performance of atmospheric optical communication systems. Turbulence-induced intensity scintillation often reduces the capability of receiver to extract and recover information contained in the modulated optical wave. Based on the Gamma-Gamma distribution, the bit error rater (BER) performance of atmospheric optical communication system using subcarrier PSK intensity modulation is investigated. The bit error rate for turbulent channel is derived for subcarrier BPSK and OOK modulation schemes. Under some conditions, comparison is given about the BER performance of the above-mentioned two modulation schemes. The influences of link character, aperture diameter, wavelength and zenith angle on the BER are evaluated. Results show that the BER is effectively reduced by increasing aperture and wavelength and increases with increasing the zenith angle. It is also shown that the BER performance of systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation.