The application of OFDM in troposcatter communication can enlarge the tropo communication capacity. Meanwhile, Inter Carrier Interference (ICI) of OFDM system is the major drawback to degrade OFDM system performance. As a result, the multi-antenna diversity techniques are introduced and the OFDM with Maximal Ratio Combining (MRC) diversity under tropo fading channel is investigated. Firstly, the tropo channel model is established, then, system ICI is investigated. OFDM system with MRC diversity under tropo channel is investigated, and the system SER is derived. Simulation results show that ICI can degrade the SER performance and introduce symbol error floor while multi-antenna diversity can solve the problem above. When normalized Doppler spread equals 0.02 and signal to noise ratio equals 20dB, dual diversity can decrease SER by two orders of magnitude and triplex diversity with four orders of magnitude.