With increased demand of high data rates with mobility, broadband wireless services have seen tremendous growth in recent past years. The mobile Worldwide Interoperability for Microwave Access (WiMAX) is based on IEEE 802.16 standard and is used for wireless Metropolitan Area Network (MAN). The IEEE 802.16 standard supports high data rate and high capacity in mobile Broadband Wireless Access (BWA). The inclusion of Multiple Input Multiple Output (MIMO) in mobile WiMAX system provides a robust platform for space, time and frequency selective fading conditions and increases both data rate and system performance. The simulation of MIMO-mobile WiMAX model is done by using MATLAB. Bit Error Rate (BER)The performance of mobile MIMO WiMAX system has been carried out using Space Time Block Code for different modulation schemes (QPSK, 8-PSK, 16-QAM, 64-QAM) under different channel conditions like AWGN, Rayleigh and Rician for different diversity schemes like Alamouti (2x1), MRC (1x2), MRC (1x4) STBC-ML (3x4). The MRC scheme is used at receiving side for processing received signal.