In this work, highly dispersed and different morphologies of Co3O4 on the surface of GO modified with polyethylenimine (PEI) were synthesized by hydrothermal method at different temperatures. Co3O4/PEI-GO exhibited better gas sensing properties for NOx gas than the pure Co3O4 and Co3O4 /GO. Especially, the Co3O4/PEIGO composite with hydrothermal at 190 degrees C for 3 h showed excellent sensitive characteristics to NO, at room temperature. The cause of good gas sensing property come mainly from the synergy between Co3O4 and graphene oxide (GO) modified with PEI, which can significantly enhance the gas sensing performance. The adsorption species of NOx on the surface of Co3O4/PEI-GO composite sensor was detectable by In Situ Diffuse Reflectance Infrared Transform Spectroscopy (DRIFT). The electronic core-shell configuration model has been established to illustrate the resistance change when the Co3O4 is exposed to oxidizing gases. Hence, the Co3O4/PEI-GO composite materials are promising in application of detecting NOx gas sensing.