The issue of gas source localization (GSL) is studied in this paper. A novel method of localizing a single gas source based on a mobile sensor network (MSN) is proposed. Compared with the existing wireless sensor network (WSN) methods, the proposed MSN method only needs a few mobile sensor nodes instead of lots of stationary sensor nodes. In addition, unlike the WSN methods, the sensor nodes in the MSN are not required to enclose the gas source. In the MSN method, each sensor node sends the measured gas concentration to a central node, in which the nonlinear least square algorithm is used to estimate the location of the gas source. The MSN moves some distance towards the estimated gas-source location, and then the central node re-estimates the location of the source and the MSN moves again. Such a process is repeated till the MSN is close enough to the real source. Gas sensors are calibrated according to the relation between the variation of output voltage and the gas concentration in order to decrease the impact of the ambient temperature. Experiments in an indoor natural ventilated environment validate the proposed method.