The performance of both primary and secondary networks in underlay cognitive radio networks (CRNs) with the help of incremental amplify and forward (IAF) relaying under imperfect channel state information (CSI) are investigated. Particularly, the interference probability at primary networks denoted by IP as well as the outage probability at secondary networks denoted by OP, are computed in the closed-form expressions. Our findings show that the impact of the imperfect CSI on the performance of IP is non-negligible. To tackle the high value of IP, reducing the transmit power at the secondary transmitters is a proper solution, it, however, will also increase the OP at the secondary networks. Thus, a simple power control coefficient is proposed to compromise the performance between two networks. Finally, Monte-Carlo simulations are provided to verify the accuracy of the proposed mathematical frameworks.