We are investigating a hydrodynamic particle separation technique that employs pinching of particles to a narrow microchannel. The particles are subject to a sudden expansion which results in a size-based particle separation transverse to the flow direction. The separation resolution and particle dispersion are measured using epifluorescence microscopy. The resolution and dispersion are predicted using a compact theoretical model. Devices are fabricated using conventional soft lithography of polydimethylsiloxane. The results show that the separa-tion resolution is a function of the microchannel aspect ratio, particle size difference, and the microchannel side- wall roughness. A separation resolution as large as 3.8 is obtained in this work. This work shows that particles with diameters on the order of the sidewall roughness cannot be separated using pinched flow fractionation.