The bubble formation in highly viscous liquids in a microfluidic flow-focusing device is highlighted. The influences of viscosity and flow rate of liquid phase on the bubble formation are studied. The glycerol water solution (94-1000 mPa s) is used as the liquid phase, and nitrogen as the gas phase. Two different stages are observed in the gaseous breakup during bubble formation: at first, the relation between the neck radius of the gaseous thread and the remaining time can be expressed as a power law; while in the final stage, the gaseous thread thins linearly with time. Under constant gas flow rate, the exponent in the power law stage is influenced by the liquid viscosity, and the thinning rate of the linear pinch-off stage decreases with the increase of the liquid viscosity. The liquid flow rate has no obvious effect on the variation of the neck radius of the gaseous thread with time in both stages. The results show that the increase in liquid viscosity enhances the confinement effect on gas-liquid interface during bubble formation. (C) 2019 Elsevier Ltd. All rights reserved.