Two artifacts were observed during collection of atmospheric ultrafine particles (UFPs; < 100 nm diameter) using an inertial filter (INF) sampler recently developed for this purpose. These artifacts were evaluated. First, the adsorption of organic and ionic gas onto a quartz fiber filter installed in the INF sampler was evaluated to provide information on the positive artifact; this information is important for accurate analysis of the UFP components. Gas adsorption by the INF sampler was similar to or less than that of other devices for collecting UFPs. The gas adsorption of organic carbon (OC) fractions, and ionic components of NH4+, NO3– and SO42– was confirmed, and the adsorption of OC1 and ionic components were highly dependent on the concentration of the gas and the UFP concentration under ambient conditions. These results suggest that it is necessary to know the concentration of the UFPs on the filter in order to evaluate the exact concentrations of UFP components under high flow rate conditions. Second, the efficiency of UFP collection on a polytetrafluoroethylene (PTFE) filter installed in the INF sampler was evaluated to clarify the negative artifact. The data confirmed that the efficiency of collection changed with changing structure such as pore size, porosity and thickness of the filter. The structure of these filters and UFPs collected on there were observed by a scanning electron microscopy. The highest particle collection efficiency (almost 100%) was obtained by installing two thick membrane PTFE filters. The collection of UFPs using a sampler comprising several filter stages is a convenient and useful method for evaluating positive and negative artifacts and for quantifying the concentration of components of UFPs.