Abstract: Nitrogen dioxide (NO₂), as one of the most concerned air pollutants, has caused serious effects on water, soil, atmosphere and human health. It is urgent to develop a portable NO₂ gas sensor that can detect low concentration of NO₂ with high response at room temperature. In this work, (111) oriented NiO nanosheets were fabricated on quartz substrate by hydrothermal method. The orientation, density, thickness of the NiO thin film were adjusted by changing the concentration of the NiCl₂·6H₂O precursor in the hydrothermal reaction. NO₂ gas sensor based on the (111) oriented NiO thin film could work at room temperature, and it can detect 10 ppb NO₂ with a response of 19%. It also has good selectivity toward NO₂.