In this paper, we derive an exact upper bound (UB) on bit error rate (BER) of the decode-and-forward (DF) cooperation protocol. The destination uses the maximum a posterior (MAP) decoder to estimate the data sent from the source. The MAP decoder is optimal in the sense that it minimizes the the end-to-end error probability. The complexity of derivation comes from the fact that however the source generates data with equal probability, the data received at the destination does not have the same a priori probability. That is because of the error that occurs in the source-to-relay link. Hence, the MAP decoding rule can not be simplified to the maximum likelihood (ML) decoding rule. The results show that the upper bound is very tight. Therefore, the closed form expression of the upper bound can be used to fully study and understand the diversity performance of the DF cooperation protocol.