Recently, heterostructured nanomaterials have attracted great attention in gas sensing applications. However, the sensing mechanism of the enhanced sensitivity of heterostructured nanomaterials remains unclear, which is not conducive to further improvements in their sensing performances. In order to detail the fundamental studies on the gas sensing mechanism of heterostructured nanomaterials and improve the room temperature NO₂ sensing properties of NiO-based nanomaterials, NiO–SnO₂ heterojunction nanocomposites were fabricated. It was found that the sensitivity of the nanocomposites was largely enhanced compared to the bare NiO. On the basis of the intrinsic characteristics of the p–n heterojunction and the band structure of the NiO–SnO₂ heterojunction, the largely enhanced room temperature NO₂ response of the nanocomposites could be attributed to two factors. One was the significantly decreased initial conductance, and the increase in the equivalent hole concentration of the nanocomposites after exposure to NO₂, associated with the effective electron transfer via the interface bonds at the heterojunction. Another was that the variation of contact potential in the nanocomposites, before and after exposure to NO₂, exerted a drastic effect on the transducer function for gas sensing. According to the differentiation in the sensitivity of the nanocomposites with different molar ratios, the important role of interface bonds in gas sensing properties was further illustrated by the dependency of the sensitivity on the interface bond number and the interface resistance. Here, we hope that this work could give us a better understanding of the gas sensing mechanism of the p–n heterojunction, and provide a proper approach for heterojunction materials to further improve their sensing performances.