Nanofiber filtration is drawing an ever-increasing attention nowadays because of its high filtration efficiency as well as low basic weight. The objective of this study is to investigate the effect of structural characteristics on filtration performance with a single nanofiber mat between two pieces of nonwoven membranes. The filtration performance of nanofiber mats was evaluated by quality factor, the ratio of aerosol filtration efficiency to pressure drop. It was found that the quality factor dropped rapidly when the average fiber diameter (d(f)) increased from 358 to 425 nm and decreased slowly from d(f) = 425 nm to d(f) = 1250 nm. This proved that gas-slip effect occurred on nanofibers with smaller diameters. Similarly, the quality factor of unimodal nanofiber mat declined as the packing density increased. Meanwhile, these data were compared with corresponding prediction of ideal mats predominantly from theoretical equations. Nanofiber mats with bimodal fiber size distributions were tested at the same condition. When compared with the unimodal nanofiber mats having the same weight-averaged fiber diameter and similar packing density, the bimodal nanofiber mats exhibited higher quality factors. Hence, the bimodal method is an effective method for the improvement of filtration performance. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128:1089-1094, 2013