WiMAX system is based on the IEEE 802.16-2005 specification and provides wireless broadband to fixed and mobile users. It adopts MIMO technology to produce a high system capacity and spectral efficiency. By coupling a robust and efficient OFDM technique with MIMO antenna diversity systems, we offer a very compelling high-speed data downlink solution for future wireless communications systems. IEEE802.16e WiMAX system proposes an adaptive digital modulation and coding schemes according to the wireless link quality. In this work, we present a Cross-Layer approach based on the adaptive Modulation and Coding Scheme (MCS) mechanism which incorporates Network layer adaptive code with the PHY layer feedback information. Results show that this design achieves better performance of the WiMAX achievable throughput and transmission efficiency than adaptive modulation and coding used only with ARQ retransmissions through STBC and SM schemes.