Abstract: The issue of optimizing energy efficiency of cooperative Automatic Repeat Request protocol (CARQ) in wireless sensor networks is studied in this paper. Energy efficiency of CARQ protocol is defined as the number of data packets received successfully at the destination node per the unit of energy cost. The expressions of energy efficiency of CARQ and traditional ARQ protocol are deduced respectively and the energy efficiency gain of CARQ protocol compared with ARQ protocol is also derived in which the energy efficiency of them are compared with each other. Simulation results show that the position of the relay node has an important effect on the energy efficiency of the CARQ protocol and the CARQ protocol provides better energy efficiency than ARQ protocol when the communication distance is bigger than the threshold of the distance. At last, a discrete optimization algorithm optimizing the modulation level is proposed, by which energy efficiency and its gain of CARQ protocol can be enhanced greatly.