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Primary user detection is the key enabling technology in cognitive radio. In this paper, a spectral entropy based detection method is proposed. More specifically, when primary user is active, the received signal of secondary user will have less spectral complexity than other times, which is measured by spectral entropy. The presence of primary user can be examined by comparing the estimated spectral entropy of signal to a threshold. Simulation results show that the proposed detection algorithm can achieve good performance in very low SNR environments. Moreover it dose not need any prior knowledge of primary signal and noise energy level.