The breakup mechanism of high-viscosity thread for droplet formation in a flow-focusing device is investigated using a high-speed digital camera. Aqueous solution of 89.5%-glycerol is used as the dispersed phase, while silicone oil as the continuous phase. The breakup process of the dispersed thread presents two categories: symmetrical rupture and asymmetrical rupture. Furthermore, the rupture behavior could be divided into two stages: the squeezing stage controlled by the squeezing pressure and the pinch-off stage controlled by viscous stresses of both phases and surface tension. Specifically, it suggests that the differences in the shape of the liquid-liquid interface and the dynamics in the two breakup processes are caused by the disparity of the strain field at the point of detachment. Moreover, the thinning rate and the dynamics of the dispersed thread change with the viscosity of the continuous phase, but are less dependent of the flow rate of the continuous phase. (C) 2015 American Institute of Chemical Engineers