The following paper investigates the secondary sum rate of MIMO two-way cognitive relay networks with multiple relays using amplify and forward (AF) strategy. The capacity of the system can be maximize by using optimum power allocation (OPA) algorithm in place of equal power allocation(EPA) among all nodes, while assuming that all node having fully channel state information (CSI). It has been considered that MIMO cognitive relay with spectrum sharing and cooperative strategy between node to maximize the secondary sum rate and enormous use of the spectrum. Our aim is to maximize the secondary sum rate and efficient spectrum use. MIMO relay is going to use amplify and forward relay technique means relay is not decode the signal received from the secondary user's. Result shows that this technique is increasing the secondary sum rate without using extra power and spectrum.