Abstract: In this paper, we first investigate the power allocation for the source and relay of Underlay cognitive radio relay cooperation (CR-RC) systems, and present the independent power allocation (IPA) and joint power allocation (JPA) schemes. Then, with the IPA and JPA, the closed-form expressions of outage probability and ergodic capacity for Underlay CR-RC systems are achieved. Finally, through the comparison of outage performance, a hybrid Interweave-Underlay CR-RC schemes is presented. In the presented hybrid Interweave-Underlay CR-RC scheme, by considering the quality of service (QoS) of both the primary users (PUs) and secondary users (SUs), we define the constraint on the outage probability of primary and secondary systems. The main idea of the presented hybrid Interweave-Underlay CR-RC scheme is: 1) The SUs send its signals with maximum power under the case where the QoS of PUs is not guaranteed even if the transmit power of SUs is zero. 2) The SUs do not transmit signal under the case where the outage probability of SUs is very high due to the severe interference from PUs. 3) When the QoS of PUs and SUs is guaranteed simultaneously, the primary and secondary users work together by sharing spectrum.