This chapter investigates performance of OFDMA based mobile WiMAX downlink Multiple Input Multiple Output (MIMO) beamforming algorithms and analyzes Signal to Interference plus Noise Ratio (SINR) distribution for the OFDMA based WiMAX uplink multi-user MIMO systems. MIMO techniques are supported in IEEE 802.16 standard to achieve array gain, diversity gain and multiplexing gain. Beamforming and beamforming plus MIMO increase transmission ranges and improve SINR for MIMO systems by reducing co-channel interferences. The beamforming-only scheme aims at interference cancelation to improve link SINR. The beamforming Plus MIMO scheme can not only exploit the array gain by using beamforming but can also achieve transmit or receive diversity gain and spatial multiplexing gain. Our simulation results show that beamforming plus MIMO significantly improves the performance by combining diversity gain and beamforming gain and alleviating the effect of antenna correlation. Therefore, the beamforming plus MIMO is a promising technique for the mobile WiMAX applications. For WiMAX uplink MIMO systems, a mathematical expression of the received SINR is derived, which provides an analysis tool for the studied WiMAX systems.