In the present work, Cu-doped ZnO composited with reduced graphene oxide (rGO) was prepared by low temperature solvothermal method and the following annealing process in the H2/Ar gas mixture.The morphology characteristic results show that ZnO nanoparticles anchored uniformly on the rGO.The as-prepared nanocomposites with different amounts of Cu dopants exhibited different sensing properties, and when the molar ratio of Cu to ZnO is 6%, the room-temperature response of nanocomposites to 2000ppm CO reach maximum (3.94).To further understand the sensing mechanism of nanocomposites, the existence of oxygen vacancies was confirmed by the EPR, and the probable sensing mechanism based on the nanocomposites was also proposed.It was believed that more oxygen vacancies in the nanocomposites facilitate more oxygen molecules adsorbed on the surface of nanocomposites, and then participate in the reaction with CO gas, leading to the higher sensitivity of the nanocomposites.Moreover, the Cu sites can also benefit the CO adsorption.Here we hope this work can help us to further explore more room-temperature CO sensors with higher sensitivity.