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In this paper, we consider the bit error rate (BER) performance analysis of a cooperative relay communication system with differential modulation and selective relaying, where the best relay is selected to forward the signals from the source to the destination. The advantage of differential modulation is that it allows noncoherent detection without requiring the full channel-state information (CSI), as obtaining CSI in a cooperative communication system at each node is a complicated and costly task. We consider a simple detect-and-forward (DetF) relay system and take into account the effect of imperfect detection at the relay. We derive the average bit error probability of the DetF relay scheme by using the closed-form relay link signal-to-noise ratio (SNR). Based on the analysis, we show that the proposed system with N relay nodes and the simple relay selection can achieve a total diversity gain of N + 1. The BER performance of the system is also evaluated by simulation. It is shown that the analysis matches well with the simulation results.