To study the effect of morphologies on the gas sensing behaviour, herein, we have synthesized three different CuO nanostructures, namely CuO nanobrick, CuO hierarchical flower-like and CuO nanochain. Hydrothermal method has been used to synthesize the CuO nanobrick and CuO hierarchical flower-like morphologies, whereas wet chemical method for CuO nanochain-like morphology. The present study reveals that all the CuO nanostructures-reduced graphene oxide (rGO) composite sensors show maximum response for NO2 gas at room temperature and intermediate humidity level (~ 50%RH). Furthermore, among all studied sensors, the CuO hierarchical flower-like/rGO sensor exhibits maximum NO2 gas-sensing response. At RT, the CuO hierarchical flower-like/rGO sensor shows response of ~ 58.1% in 30 s for 20 ppm NO2 which is ~ 1.7 times of CuO nanobrick/rGO sensor and almost twice of CuO nanochain/rGO composite sensor. Moreover, the gas sensing behaviours of composite samples synthesized with different weight (wt) ratios of rGO to CuO hierarchical flower-like for NO2 gas have also been studied to find optimum weight ratio of rGO and CuO hierarchical flower-like for which best sensing performance is observed. Consequently, CuO hierarchical flower-like with 25 wt% of rGO exhibits the highest response. All sensors show good reproducibility when tested for five successive cycles of 20 ppm NO2 and outstanding selectivity for NO2 gas. Furthermore, the sensing mechanism has been discussed in this paper by considering the role of morphology. Our work suggests that morphology of CuO and adding appropriate amount of rGO are crucial factors and influence the sensing behaviour of NO2 sensor.