This work reports the breakup dynamics for slender droplet formation in shear-thinning fluids in flow focusing devices consisted of respectively 600 mu m and 400 mu m square microchannels. Silicon oil droplets are generated in non-Newtonian shear-thinning fluids-polyacrylamide (PAAm) aqueous solutions. The thinning of the thread of the dispersed phase during droplet formation can be characterized by a power law relationship with the remaining time before the final pinch-off, with an exponent of 1/3 in the non universal collapse stage, followed by an exponent of 1 in the final universal pinch-off stage. The dependence of the pre-factors on the viscosity ratio of both phases and the capillary number is discussed. A scaling law is proposed to predict the size of droplets formed in shear-thinning fluids in such devices by taking into account the rheological property of the non-Newtonian fluids as well as the hydrodynamics. (C) 2016 Elsevier Ltd. All rights reserved.