In differential cooperative diversity systems，the traditional maximal ratio combiner (MRC) needs the
second-order statistics of all channels as the weights of the combiner． Meanwhile，a selection combiner (SC) can form a decision variable based on the received signals，and the link with the maximum magnitude of the decision variable is chosen for non-coherent detection． In this paper，the performance of a differential amplify-and-forward system employing a SC is analyzed． Then，an exact analytical expression of the average bit error rate (BER) and the outage probability are derived by means of the moment generating function (MGF) method and the optimal power allocation point is put forward． Theoretical analysis and simulation results demonstrate that the performance of the differential amplify-and-forward system employing the SC and the MRC is better than that of direct transmission， and in comparison with the MRC scheme，the SC scheme without any channel state information reduces the complexity and achieves a similar performance．