Abstract: In cognitive relay networks with underlay spectrum sharing, the transmission power of secondary users is strictly confined, for protecting primary users′ quality of service (QoS). Hence, the transmission performance of secondary users is difficult to be guaranteed. Outage probability is one of the most important measurements to evaluate system performance. In this paper, the outage performance is analyzed in the multiplerelay cognitive radio networks, which employs “selectivebest relay amplifyandforward (AF)” scheme. And the exact outage probability expressions in both broadcasting phase and relaying phase are obtained. Moreover, the joint probability density function is derived when the received signaltonoise ratios (SNRs) are dependent. Based on that, the lower bound on outage probability is given, in the case that interference power to primary users is limited. Finally, through numerical simulation, the theoretical analysis is validated.