Functional adjustment of graphene with metal oxide can in fact progress the affectability of graphene-based gas sensors. However, it could be a huge challenge to upgrade the detecting execution of nitrogen dioxide (NO2) sensors at room temperature. The ultrasmall size of nanocrystals (NCs) and copious defects are two key variables for moving forward gas detecting execution. Herein, we provide an effective strategy that the hydrothermal reaction is combined with room-temperature oxygen plasma treatment to prepare Co3O4 NCs and reduced graphene oxide (RGO) nanohybrids (Co3O4-RGO). Among all of Co3O4-RGO nanohybrids, Co3O4-RGO-60 W exhibits the most superior NO2 sensing properties and achieves the low-concentration detection of NO2. The sensitivity of Co3O4-RGO-60 W to 20 ppm NO2 at room temperature is the highest (72.36%). The excellent sensing properties can mainly depend on the change in the microstructure of Co3O4-RGO. Compared with Co3O4-RGO, Co3O4-RGO-60 W with oxygen plasma treatment shows more favorable properties for NO2 adsorption, including the smaller size of Co3O4 NCs, larger specific surface area, pore size, and more oxygen vacancies (OVs). Especially, OVs make the surface of NCs have a unique chemical state, which can increase active sites and improve the adsorption property of NO2. Besides, the agreeable impact of the p-p heterojunction (Co3O4 and RGO) and the doping of N molecule contribute to the improved NO2 detecting properties. It is demonstrated that the Co3O4-RGO-60 W sensor is expected to monitor NO2 at room temperature sensitively.