In this work, a novel intermediate-size porous silicon (intermediate-PS) gas sensor was prepared successfully by electrochemical etching method. The morphology and geometry of PS samples were observed using field emission scanning electron microscope (FESEM). The surface chemical bonds were determined by Fourier transform infrared (FTIR) spectroscopy. It is found that PS showed typical n-type semiconductor behavior. Furthermore, it exhibited good response value, excellent repeatability and fast response-recovery characteristic when exposed to NO2 gas at room temperature. Compared with other microstructures of PS, intermediate-PS showed much better gas-sensing properties because high special surface area provided more adsorption sites, and unique structural properties dramatically increased rates of gas diffusion. In addition, possible NO2-sensing mechanisms and potential applications were also discussed. (C) 2012 Elsevier B.V. All rights reserved.