Rheological properties of some polymer solutions in both the linear and non-linear regions have been investigated. The solutions include poly(vinyl alcohol) solution (with and without sodium borate), hyaluronic acid solution, and xanthan gum solution; some of which form microstructures depending on their deformation history, and thus can be classed as complex fluids. Among the theological properties, the large amplitude oscillatory shear (LAOS) behavior was found to be very sensitive to the interactions or the shear-induced formation of microstructures. Depending on the interactions between the microstructures, it was found that there exist at least four types of LAOS behavior: type I, strain thinning (G', G" decreasing); type II, strain hardening (G', G" increasing); type III, weak strain overshoot (G' decreasing, G" increasing followed by decreasing); type IV, strong strain overshoot (G', G" increasing followed by decreasing). It is suggested that the LAOS behavior can be effectively used as a tool for classifying the complex fluids.