In this work, graphitic carbon nitride nanosheets decorated like nickel oxide microspheres (NiO/g-C 3 N 4 , Ni/CN) were successfully synthesised by a simple hydrothermal method. The morphologies of the as-prepared samples were analysed by scanning electron microscopy and the diameter of the microspheres was found to be about 2.5 μm. At an optimum working temperature of 240°C, the response to 500 ppm CO is 2.729 for the Ni/CN-10 (10 wt% g-C 3 N 4 modified NiO) sensor, which is much higher than those of the NiO sensor (1.408). Besides the higher response, the Ni/CN sensors also showed good repeatability and stability for CO detection. Excellent gas sensitivity properties can be attributed to the p–n heterojunction formed at the interface, which can provide more potential active sites for surface reactions. In addition, the sensitisation of g-C 3 N 4 also plays an important role in the improvement of CO-sensing performance.