The main focus of current research and development for the next-generation wireless local area network (WLAN) communication systems is to enhance the link throughput and channel capacity. In this paper, the performance analysis of the ongoing next-generation WLAN standard, IEEE 802.11n high throughput WLAN PHY layer is presented. The design criteria is based on a 4times4 MIMO-OFDM scheme using 64-QAM (quadrature amplitude modulation) technique under Rayleigh frequency-selective fading and flat fading channels. Simulation results show that a significant performance gain, 4-7 dB, depending on the channel selection, can be achieved when 1/2 space-time block code (STBC) rate is applied with respect to bit-error-rate (BER). Furthermore, performance gain of 7 dB is achieved when the 4times4 system using 1/2 STBC is compared with a 2times2 system using STBC code rate 1.