Multiple input multiple output (MIMO) is used in order to decrease the effects of atmospheric attenuation and turbulence on free space optical communication (FSO). It is assumed that the intensity modulation direct detection (ID/DD) with on-off keying (OOK) is used in the FSO communication systems. The channel is memoryless, stationary ergodic, identically independent and distributed with additive white Gaussian noise (AWGN). The perfect channel state information (CSI) is available to both the transmitter and the receiver. Based on the hypothesis, the multiple input multiple output channel model and systems model are established under the integrated atmospheric effects. The closed form expressions of ergodic capacity and outage probability of FSO system with the equal gain diversity combined technique are derived. The influences of various weather conditions and turbulence strength on space optical communication link are analyzed. The results indicate that the performance of the communication system is increased and the atmospheric effect is decreased with the increase of the number of transmit aperture and receiving aperture. Considering the complexity and the increasing performance of the system, 2 transmission apertures and 2 receiving apertures are more suitable for FSO.