Microfluidics-based particle separation has attracted much attention in a wide range of chemical, environmental, and biological applications. However, most of the existing methods require complex channel designs to generate inertial flows or external forces such as electric fields. In this work, we demonstrate a facile particle separation technology with extremely simple straight channel geometry not relying on any external force. In viscoelastic flow, larger particles are enriched downstream of a straight channel in a self-modulated manner by sheathless elasto-inertial focusing mechanism (Yang et al., Lab Chip, 2011, 11, 266-273). We evaluated the performance of a microfluidic separator based on this mechanism, and found significant effects for polymer and particle concentrations, as well as flow rate. In particular, we determined an upper limit for the polymer concentration, which was attributed to the occurrence of shear-thinning behavior, and we found optimal flow rates for the separation. In addition, we found that particle-particle interaction plays an important role in the separation process and the purity of separated particles is gradually degraded with increasing particle concentration. This work will contribute to the design of microfluidic particle separators and the fundamental understanding of particle dynamics in polymer solutions flowing through confined geometries. (C) 2014 Elsevier Ltd. All rights reserved,