One of the methods to solve the problem of spectrum demand is spectrum sharing and can be achieved using Cognitive Radio (CR) network. The problem in CR network is physical layer security. The physical layer security can be improved using relays in the secondary network. There are two types of relay slection schemes which are Single Relay Selection (SRS) and Multiple Relay Selection (MRS) schemes. In SRS and MRS schemes, the coordination between primary and secondary network is lost due to mutual interference which results in imperfect Channel State Information (CSI). In this paper,Transmit Antenna Selection (TAS) scheme in MIMO CR network has been proposed for improving the physical layer security compared to SRS and MRS schemes in CR network. The physical layer security is analyzed through Security Reliability Tradeoff (SRT) in terms of Outage Probability (OP) and Intercept Probability (IP) for SRS, MRS and TAS schemes. Simulation results show that SRT performance of TAS scheme in MIMO CR network is better than SRS and MRS schemes. © 2016 IEEE.