The tremendous growth in Wireless Technology in the field of communication is characterized with increased data speed with high accuracy. In order to satisfy the requirements of a data rate, it has greater complications. These requirements are conquered with Multiple-Input-Multiple-Output (MIMO) communication system. This system has multiple and certain number of transmitting and receiving antennas. Adaptive MIMO system has various conjuction of multiple transmitting and receiving antennas with Orthogonal Space Time Block Codes for encoding, which helps in improving accuracy and high data rates. Adaptive system will control with either one, two, three, or four transmit and receive antennas. The Orthogonal Space-time block coding (OSTBC) encoder by accepting Alamouti code will encode the instruction symbols from the Quadrature Phase Shift Keying (QPSK) modulator. QPSK Demodulator is used to demodulate the output of the OSTBC Combiner. The Frame Error Rate (FER) for three transmit and two receive antennas and the frame error rate for four transmit and two receive antennas is estimated. From the simulation results it is observed that the three transmit and two receive antennas are better.