The planar contraction/expansion multiple-stream flow instability in microchannels and mixing performance of two-dissimilar polyethylene-oxide (PEO) fluids were examined. Experimental results indicate that fluid viscosity and low fluid inertia were insufficient to induce flow instability. The viscoelasticity of the fluids is essential for flow instability and for mixing enhancement, which can be typified by Deborah number (De, elastic / viscous forces). This chaotic flow instability was generated at very high Elasticity number (El = De / Re) and Reynolds number is no longer relevant. These investigations showed that flow instability strongly depended on the ratio of viscoelasticity of the two streams and the viscoelastic energies of the flow streams. A governing parameter, Deratio = Demain / Deside (where Demain and Deside are De associated with the main and the side streams respectively), was employed to characterize the dynamical flow behaviors. A Deratio— Demain operating space summarizes the multiple-stream flow instability and mixing performance.