Secure data transmission in future high-capacity high-coverage multi-tier hierarchical networks, for which cognitive radio(CR) has emerged as an essential recipe, is of utmost importance. This paper investigates the secrecy outage performance of selection combining(SC) in CR networks(CRNs) over Rayleigh fading channels. In a single-input multiple-output(SIMO) wiretap channel, a secondary user transmits confidential messages to another secondary user, which is equipped with( 1)B Bn n ≥ antennas. Meanwhile, a passive eavesdropper, which is equipped with( 1)E En n ≥ antennas, intends to overhear the messages. Both the legal receiver and the eavesdropper adopt SC scheme to process the received multiple signals. The secondary transmitter uses the underlay strategy to guarantee the quality of service of the primary user without spectrum sensing. Compared to the work proposed by Maged Elkashlan et al. in Ref. [1], we present an alternative method to derive the closed-form expression for the secrecy outage probability(SOP) and develop a simplified SOP when the maximal transmit power at the secondary user is sufficiently high. Our results reveal the impact of the primary network on the secondary network with a multi-antenna wiretap channel and simulations are conducted to validate the accuracy of our proposed analytical models. Secure data transmission in future high-capacity high-coverage multi-tier hierarchical networks, for which cognitive radio (CR) has emerged as an essential recipe, is of utmost importance. This paper investigates the secrecy outage performance of selection combining (SC) in CR networks (CRNs) over Rayleigh fading channels. In a single-input multiple-output (SIMO) wiretap channel, a secondary user transmits confidential messages to another secondary user, which is equipped with (1) B Bn n ≥ antennas. Meanwhile, a passive eavesdropper, which is equipped with (1) E En n ≥ antennas, intends to overhear the messages. Both the legal receiver and the eavesdropper adopt the SC scheme to process the received multiple signals. The secondary transmitter uses the underlay strategy to guarantee the Compared to the work proposed by Maged Elkashlan et al. in Ref. [1], we present an alternative method to derive the closed-form expression for t he secrecy outage probability (SOP) and develop a simplified SOP when the maximal transmit power at the secondary user is sufficiently high. Our results reveal the impact of the primary network on the secondary network with a multi-antenna wiretap channel and simulations are conducted to validate the accuracy of our proposed analytical models.