In this paper, we demonstrate the performance analysis of an underlay MIMO cognitive radios employing space time block coding at secondary’s user transmitter (SU-Tx) and generalized selection combining at secondary’s user receiver. The transmit power strategy enforces the SU-Tx to adapt its power to be transmitted so as to protect primary user’s receiver from harmful interference in order to satisfy primary user outage constraint. We derive closed form expressions for average post processing signal to noise ratio and ergodic capacity. Moreover, approximate expression for average bit error rate is also obtained. Based on the achieved results, we show that the average post processing SNR, Ergodic Capacity and average BER improve as number of best branches to be combined increases. Finally analytical results are affirmed through Monte Carlo simulations.