Droplet formation and breakup dynamics in either dripping or jetting regimes in microfluidic flow-focusing devices were investigated experimentally. More viscous liquids were dispersed into less viscous liquids in square microchannels with 600 and 400 mu m wide, respectively. In the dripping regime, for low viscosity ratio of both phases, the variation of the minimum width of the dispersed thread with the remaining time could be scaled as a power-law relationship with exponent related to initial conditions, and the droplet size could be correlated with the flow rate ratio of both phases and the capillary number of the continuous phase; while for high viscosity ratio of both phases, the dispersed thread experiences a linear thinning procedure, and the droplet size can be scaled with the Weber number of the continuous phase. In the jetting regime, the stable jet width was affected by the viscosity ratio and flow rate ratio of both phases. And the relationship between the droplet size and the flow rate ratio of both phases could be scaled as a power-law relation with the exponent dependent on the viscosity ratio of both phases. The analysis of the mode of the maximum instability of the oil jet indicated that the most unstable instability is related to the viscosity ratio of both phases and is almost independent on the flow rate ratio of both phases. (C) 2012 Elsevier Ltd. All rights reserved.