Particle inertial loss in bends of a circular cross section was studied experimentally in a low-pressure environment down to 0.2 Torr. Uniform aerosols across the bend inlets were produced by a new method developed by Sato et al. (2002a). The deposition efficiency was obtained by measuring the mass of deposited ammonium fluorescein particles with a fluorometer. The ranges of the Dean number and the curvature ratio (the ratio of the radius of curvature to the tube radius) were 1.8-2.2 and 3.6-4.7, respectively. The flow Reynolds number varied from 3.4 to 4.8. The results agreed with the general trend shown numerically by Tsai and Pui (1990), i.e., the deposition efficiency increases with increasing Stokes number and Dean number, and with decreasing curvature ratio. However, the experimental Stk50-the Stokes number at 50% deposition efficiency - was larger than that predicted by Tsai and Pui's empirical equation. The difference was attributed to the fact that their equation was obtained using the results for Dean numbers ranging from 38 to 869 and thus failed to predict the Stk50 for a smaller Dean number. A new analytical equation for Stk50 as a function of the Dean number and the curvature ratio was obtained. The proposed new equation can be used to calculate the Stk50 for Dean numbers near unity.