In order to improve the transmission rates of cognitive radio (CR) and primary user (PU), the cooperative communication scheme between CR and PU and the optimal sub-band power allocation are proposed respectively. If the presence of the PU is detected, the scheme allows CR to use half of the frame to transmit data while using the left half to help PU with cooperative transmission. The proposed power allocation based on the scheme can make CR achieve higher rate without influencing PU. Simulation results show that compared with the conventional non-cooperative communication, the proposed scheme can make improvements on the rates of both PU and CR.