SUMMARY Increasing the transmitting power will improve the system performance, but it will also increase the potential intercepted risk. Thus, the research of security‐reliability trade‐off (SRT) is valuable. In this paper, we investigate the physical layer security in cognitive radio network (CRN), which consists of a source node, a destination node, an eavesdropper, and multiple relays. The security and reliability are measured by intercept probability (IP) and outage probability (OP), respectively. Different from the previous researches, a more actual scenario is studied for the CRN in the Rayleigh fading environment; that is, the eavesdropper can eavesdrop on the secondary source (SS) and the relays. Besides, we consider the probability of false alarm and missing alarm and study their impact on the OP and IP for direct transmission (DT) and relay selective transmission (RST). The closed‐form expressions of OP and IP show that the false alarm affects the OP and the missing alarm has influence on the OP and IP. Furthermore, we accumulate the sum of the OPs for the RST in the two time slots and compare them with the DT, when we explore the OP. In addition, since the signals may be both intercepted in the two stages of RST, we analyze the IPs in the two time slots respectively and compare them with the DT separately. The simulation results show that the RST scheme can greatly improve the security and reliability by increasing the number of relays compared with the DT scheme.

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