The objective of this study is to investigate the filtration of nano-aerosols (50-500 nm) using nanofiber filter for strong diffusion capture wherein Peclet number Pe drops to single digit and even below unity. The effect of strong slip flow or transitional flow, with the dimension of the flow domain comparable in order of magnitude to the distance of mean-free-path of air molecules (i.e. Knudsen number Kn(f) of order one), has also been investigated with nanofiber diameter reducing below 100 nm. Also, we examined implications due to use of very small diameter nanofibers. Nanofiber filters are made by electrospinning Nylon 6 (N6) nanofibers on microfiber substrate. The diameter of nanofibers electrospun from 12, 20, and 24% N6 solution are 94,185, and 220 nm, respectively (corresponding Kn(f) of 1.44, 0.71, and 0.62). Previous studies showed that the conventional semi-empirical correlation as developed by Payet et al. [1] on single fiber efficiency due to diffusion mechanism (eta(D)) as developed from moderate slip flow assumption offers good agreement with experimental results even when Kn(f) increases to 0.65 (i.e. transition flow regime) over Pe starting from 40 and dropping to 6. In this study, under large slip flow (Kn(f) equals to 1.44), we show that Payet's correlation on eta(D), deviating from measurements when Pe is below 10 and the deviation gets worse when Pe drops below unity. Therefore, Payet's correlation, pertaining only to moderate diffusion capture, underpredicts the single fiber capture efficiency for nano-particle or ultrafine particle (sized below 100 nm according to ISO Standard [2]), which may lead to respiratory [3] and cardiovascular [4] problems or diseases. As such, an improved empirical correlation on eta(D), should be established based on new measurement to accurately assess the filter performance on these important particles for selection and sizing of filters in exercising preventive measures. In the study of effect of fiber diameter on capture efficiency and pressure drop, reducing d(f) from 185 to 94 nm facilitates the filtration of 50-500 nm nano-aerosol, but unfortunately elevates the pressure drop (Delta P). The benefit-to-cost effect is judged by the quality factor (QF) which should be higher for filter with better performance. The QF of a filter with 185-nm nanofibers is higher than the one with 94-nm nanofibers when filtering 50-90 nm nano-aerosol, and vice versa when filtering 100-380 nm nano-aerosol. Filtration efficiency increases when nanofiber basis weight (W) increases from 0.042 to 0.333 g m(-2). However, the QF drops rapidly when W increases from 0.042 to 0.085 g m(-2), and decreases slowly from W = 0.085 g m(-2) onwards to W = 0.333 g m(-2). As such, for a high performance filter a multilayer filter formed from stacking up of a series of low basis weight nanofiber layers, each independently supported, is more favorable over using a single-layer nanofiber filter with the same total basis weight. (C) 2011 Elsevier B.V. All rights reserved.