Solid-liquid two-phase flow in a finite Reynolds number range (2 < Re < 12), transporting neutrally-buoyant microspheres with diameters of 6, 10, and 16 pun through a 260-mu m microcapillary, is investigated. A standard microparticle-tracking velocimetry (mu-PTV) that consists of a double-pulsed Nd:YAG laser, an epi-fluorescent microscope, and a cooled-CCD camera is used to examine the flow. The solid particles are visualized in view of their spatial distributions. We observe a strong radial migration of the particles across the flow streamlines at substantially small Re. The degree of particle migration is presented in terms of probability density function. Some applications based on this radial migration phenomena are discussed in conjunction with particle separation/concentration in microfluidic devices, where the spatial distribution of particles is of great importance. In doing so, we propose a particle-trajectory function to empirically construct the spatial distribution of solid particles, which is well correlated with our experimental data. It is believed that this function provides a simple method for estimating the spatial distribution of particles undergoing radial migration in solid-liquid two-phase flows. (C) 2010 Elsevier Ltd. All rights reserved.