In this work, porous silicon with intermediate-sized pores (intermediate-PS) was prepared by using galvanostatic electrochemical etching method and the effect toward sensing response characteristics of NH3 gas was also studied. The morphology and surface chemical bonds of intermediate-PS were characterized by using field emission scanning electron microscope (FESEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The results showed the intermediate-PS microstructure can be significantly modulated by the etching current density. Moreover, the freshly prepared intermediate-PS surface could achieve reliable passivation after storage in ethanol. Furthermore, the gas-sensing measurements of the intermediate-PS sensors were carried out versus different concentrations of NH3. The PS sensor exhibited good NH3-sensing performances at room temperature owing to its unique microstructure features, including large specific surface area and highly ordered pore channels. In addition, the conceivable pore formation mechanism as well as gas sensing mechanism was also discussed. (C) 2013 Elsevier Ltd. All rights reserved.