The elasto-migration of microparticles across a streamline induced by elasticity and inertia in microscale pipe flow of viscoelastic fluids was investigated using a holographic technique. The effects of blockage ratio (d/D), flow rate (Q), and entry length (L) on particle migration induced by fluid elasticity were evaluated. Single-line particle focusing was demonstrated for an elasticity-dominant fluid. Furthermore, shear-thinning effect on particle migration was evaluated. Finally, we propose the focusing number, a non-dimensional parameter, to estimate the focusing state of particles in elasticity-dominant fluids. This criterion can be used to accurately estimate the design parameters, such as rheological properties, channel length, and particle diameter, for microfluidic devices using elasto-migration.