Flow observations of shear-thinning aqueous solutions of polyacrylamide through a 2:1 circular contraction were conducted to investigate the mechanism and structure for instability of circular entry flow of rather dilute polymer solutions as well as the types of unstable flow patterns. The visualization experiments have confirmed that flow instabilities are strongly affected by fluid elasticity and inertia. For weakly elastic fluids such as 0.1 wt.% solution, the Goertler vortices occur around the salient corner owing to the reduction of the curvature radius of the main flow due to interaction between fluid elasticity and inertia, and they move back and forth along the cylindrical wall as the elasticity number decreases. For moderately elastic fluids such as 0.2 wt.% solution, salient-corner vortex length is varied along the cylindrical wall and large vortex region moves in the circular direction. For highly elastic fluids such as 0.3 wt.% solution, fluid near the tube center exhibits buckling-like behavior in a solid-like manner in the region just upstream of the contraction entrance owing to high shear-thinning and extensional properties of more concentrated solutions. Furthermore, the present work is the first to observe the flows near the entrance face and the cylindrical wall in circular entry flow visualizations, and it is confirmed that the observation of these flow patterns is a useful and fundamental technique to study entry flow instabilities.