We have synthesized pristine SnO2 and NIO/SnO2 hierarchical structures by a facile hydrothermal method. Structural and morphological characterizations of the obtained NiO/SnO2 samples were systematically studied using XRD, SEM, TEM and XPS. Meanwhile, gas-sensing properties of the as-fabricated pristine SnO2 and NiO/SnO2 sensor devices were investigated toward ppb-level acetone detection. The measured results show that the Ni1Sn3 gas sensor can reach a response of 20.18 toward 50 ppm acetone under optimum operating temperature (300 degrees C), which is about 3.3 times higher than that of pristine sensor. Moreover, the Ni1Sn3 gas sensor also exhibits fast response/recovery time (2 s/9 s), low detection limit (10 ppb) and good selectivity. We attribute the enhanced gas-sensing properties of NiO/SnO2 hierarchical structures to the synergetic effect as well as p-n heterojunction between the NiO and SnO2 structures.