The goal of a present study is to investigate the effects of the generalized Newtonian fluids on the threshold of the transition from flow symmetry to its asymmetry for the flow through a 1:3 planar Sudden expansion. We consider Purely Viscous shear-thinning fluids covering a range of the index n of Power law model n = 0.6, 0.8 and compare them with the Newtonian fluid (n - 1.0). Sudden expansion fluid flow is studied numerically by solving the two-dimensional momentum equations along with the continuity equation and the Power law rheological model. We report a systematic results in a range of generalized Reynolds number 10 <= Re-gen <= 150 with a focus on its critical value. The subsequent effects are investigated by means of a different flow parameters, i.e. generalized Reynolds number (Re-gen), modified Reynolds number (Re-gen) and the Reynolds number defined in accordance with the inlet wall viscosity (Re-gen). Results indicate that the shear-thinning Viscous behaviour increases the onset of bifurcation phenomena and the critical value of Reynolds number. (C) 2008 Elsevier B.V. All rights reserved.