In this paper, we propose a bit error rate (BER) based algorithm for decode and forward (DF) in cooperative orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithm is issued from the prediction of the BER at the output of the channel decoder using an effective exponential signal to noise ratio (SNR). This approach offers a gain of about 2.5 dB in the single-relay case and 16-QAM modulation compared to the SNR based approach and about 0.35 dB in the case of 3 relays. The proposed technique is then a very promising candidate for DF in cooperative networks.