An inertial filter (INF) sampler can collect ultrafine particles (UFP) uniformly onto a 47 mmφ quartz filter with a low pressure drop (20–30 kPa) at 40 L/min. However, gas adsorption may influence chemical composition of UFP as a positive artifact during sampling period owing to very low mass concentration of UFP. In this study, we evaluated organic gas adsorption onto the quartz filter settled in the INF sampler during collection of atmospheric UFP in summer and winter to elucidate the influence of seasonal temperature and relative humidity. The filter samples collected by the INF sampler were analyzed for organic carbon (OC), elemental carbon (EC) by a thermal/optical carbon analyzer following the IMPROVE method.As a result, the artifacts of OC ranged from 33.0±11.7% in summer to 27.8±12.0% in winter, a large difference in the OC concentration was not observed between the seasons. The artifacts for each OC fraction showed that OC1 and OC4 were almost not adsorbed onto the filter but OC2 and OC3 were adsorbed about 40%, indicating that high boiling point components were adsorbed onto the filter at the high flow rate condition. From these results, it was thought that the gas adsorption was relatively low for the mass concentration of UFP and the INF sampler can be a useful device for UFP collection.