Orthogonal frequency division multiplexing (OFDM) has been proved as the dominant modulation technique for 4G communication. The use of relaying improves the BER performance of OFDM system. In this paper the BER performance of AF relaying with average power constraint (APS) is derived for an OFDM system. Also how frequency diversity scheme improves the BER performance in single antenna AF relay based OFDM system is discussed properly.