We measure the effect of an imperfect channel estimation and carrier frequency offset on the performance of an OFDM-based wireless local area network (WLAN) system. For the performance evaluation, the average bit error rate (BER) expression is derived in the presence of imperfect channel information, including both inherent estimation error and imperfect windowing error, derived in a mismatched channel separation. From numerical results, the inherent channel estimation error is found to be a dominating term in the system performance, compared to the other imperfections. The provided results work for the performance evaluation of the OFDM-based WLAN physical layer (PHY).