In this work, particle lateralmigration in sample-sheath flow of viscoelastic fluid and Newtonian fluid was experimentally investigated. The 4.8-mu m micro-particles were dispersed in a polyethylene oxide (PEO) viscoelastic solution, and then the solution was injected into a straight rectangular channel with a deionised (DI) water Newtonian sheath flow. Micro-particles suspended in PEO solution migrated laterally to a DI water stream, but migration in the opposite direction from a DI water stream to a PEO solution stream or from one DI water stream to another DI water stream could not be achieved. The lateral migration of particles depends on the viscoelastic properties of the sample fluids. Furthermore, the effects of channel length, flow rate, and PEO concentration were studied. By using viscoelastic sample flow and Newtonian sheath flow, a selective particle lateral migration can be achieved in a simple straight channel, without any external force fields. This particle lateral migration technique could be potentially used in solution exchange fields such as automated cell staining and washing in microfluidic platforms, and holds numerous biomedical applications.