A class of formaldehyde (HCHO) gas sensors with a high response were developed based on ordered mesoporous NiO, which were synthesized via the nanocasting route by directly using mesoporous silica as the hard template. A series of mesoporous NiO with different textural parameters such as specific surface area, pore size, pore wall thickness were achieved by selecting mesoporous silica with different pore sizes as templates. The gas sensing properties for formaldehyde (HCHO) of the NiO specimens were examined. The results show that this mesoporous NiO possesses a much higher response to HCHO even at low concentration levels than the bulk NiO, and a larger specific surface area and pore size as well as thinner pore walls would be beneficial for enhancing the sensing properties of NiO.