In this work, we investigate the use of a new low-cost gas sensor based on porous silicon to detect low concentration of NO2 at room temperature. In this work, we focused on the gas sensing properties of two different types of porous silicon (PS) prepared by a simple chemical method namely nano-porous silicon (nano-PS) and macro-porous silicon (macro-PS). The prepared sensors are characterized by different pores size, thicknesses and porosity. Electronic properties of the synthesized structures were studied by measuring the effective minority carrier lifetime using a µPCD lifetime tester. Structural properties and morphology of PS samples were also presented using Fourier transforms infrared (FTIR) spectroscopy and scanning electron microscope (SEM). The gas sensing properties of nano-PS and macro-PS were performed at room temperature for NO2 gas. Macro-PS and nano-PS sensors exhibit different behaviors attributed to the difference in their morphologies. The tested sensors exhibit considerable sensitivity at room temperature to low concentration of NO2 (4 ppm) during response time up to 45 s.