Abstract: This paper presents an improved cross-layer design which combines adaptive modulation and coding (AMC), cooperative diversity(CD) at physical layer (PHY) and the hybrid automatic repeat request (HARQ) protocol at data link layer (LL) to enhance the spectral efficiency in cooperative communication system. Only when the destination node (D) is unable to decode a packet directly from the source node (S) correctly, or the sourcedestination channel is suffering deep fading, the relay node (R) forwards the data from the source to the destination on receiving a feedback signal of a little bits. We first derive a closed form expression for the spectral efficiency of the joint cooperative HARQ-AMC under Rayleigh fading channel and decode-and-forward (DF) scenarios, then, aiming at maximizing the spectral efficiency, we search for the optimum AMC scheme by a computer algorithm. Simulations are done to verify the method. Numerical and simulation results show that the proposed scheme can further improve the spectral efficiency at medium to low signal-to-noiseratio (SNR) scenarios.