1D NiO-SnO2 nanofibers with p-n heterostructure were synthesized by electrospinning with post-synthetic heat treatment. The morphology and composition were characterized by scanning electron microscope, X-ray diffraction, and energy dispersive X-ray spectrometry. A possible growth model was proposed to describe the formation of hierarchical NiO-SnO2. The gas sensors based on NiO-SnO2 exhibited p-type response to acetone. The excellent acetone sensing properties may be attributed to numerous p-n junctions between NiO and SnO2 nanograins as well as the unique architecture. The changes of energy level and space charge layer of NiO-SnO2 heterojunction nanofibers when exposed to acetone are described in detail.