Abstract: Recently, the microchannel has attracted wide attention due to its safety, reliability and high mixing performance. Most of the current researches focus on Newtonian fluids, but the liquids used in practical applications are often elastic and viscous. Therefore, the planar laser-induced fluorescence (PLIF) technique was used to visualize viscoelastic fluid (polyethylene oxide solution) in cross-shaped channels, focusing on the influence of Reynolds number, channel sizes and polymer solution mass fraction on flow regimes, oscillatory characteristics and mixing effect. For Newtonian fluid (pure water), the separation flow, steady engulfment flow, vortex shedding oscillations and unsteady engulfment flow were found with the increase of Reynolds number (20<Re<500) in all cross-shaped channels (500 μm, 6 mm and 1 cm). For viscoelastic fluid, as the polymer solution mass fraction increased (0.01%≤w≤0.30%), the fluid elastic effect was enhanced, where an inertioelastic unsteady oscillation occurred at low concentrations which enhanced mixing, and an elastic-dominated unsteady oscillation emerged at higher concentrations within the microchannel.The elastic-dominated unsteady oscillation had periodic characteristics at low Wi and Re. As Wi and Re increased, oscillation period decreased and the flow tended to be irregular. For the 6 mm and 1 cm cross-shaped channels, the critical Reynolds number of vortex shedding oscillations and the unsteady engulfment flow increased with the increase of concentrations, and the unsteady engulfment flow disappeared at higher concentrations.