Hierarchical NiO nanomaterials doped by 0.5, 1.0, 1.5 and 2.0 at% Sn2+ ions were synthesized through a facial hydrothermal route. The isovalent Sn2+ ions were selected as dopants and occupied the interstitial spaces of NiO lattice. Such obtained NiO specimens were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) etc. and used as sensing materials for gas sensors. The comparative sensing property tests among these Sn2+ doped NiO sensors demonstrated that 1.5 at% Sn2+-NiO microspheres showed the highest response (∼25.2 ± 10%) to 200 ppm xylene, which was about 14-fold higher than that of the pure NiO. Moreover, the humidity-independent performance of the 1.5 at% Sn2+-NiO sample was excellent, such as low detection limit (1.1-500 ppb) and good selectivity toward xylene at high relative humidity (90% RH). The gas sensing mechanisms for the improved sensing performance were also discussed.