The transmission Bit Error Rate(BER) of Multiple-Input-Multiple-Output Differential Chaos Shift Keying(MIMO-DCSK) communication systems under Rayleigh fading channels are analyzed. Gaussian Approximation(GA) method based on the Central Limit Theory(CLT) is presented. The simulation results show that the consistency between GA BER curves and actual simulated curves enhances with an increasing spreading factor. Compared with DCSK, the BER performance of MIMO-DCSK is greatly improved by the increasing number of transmitting and receiving antennas. And the increase of the number of receiving antennas can get more gain than increasing transmitting antennas. With the increase of transmitting antennas, the non-orthogonality of the signals among different antennas leads to a deviation between GA BER curves and simulation curves, which will reduce after the spreading factor increasing.