This article reports the dynamics and mechanism for N-2 bubble formation in highly viscous glycero-lwater mixtures in a flow-focusing device by using a high-speed digital camera. The evolution of the volume for the gaseous thread during bubble formation is highlighted. A square microchannel with 400 mu m x 400 mu m is used. The bubble formation process can be divided into an expansion stage and a breakup stage. The volume of the gaseous thread increases linearly with time in the two different stages, however, the growth rate in the breakup stage is always greater than that in the expansion stage. The growth rate for the evolution of the gaseous thread is controlled by the capillary number and the gas-liquid flow rate ratio, with varied exponents for the two stages. The turning point of the two stages occurs at the moment when the width of gaseous neck reaches its maximum, and the dimensionless time at the turning point is related to the gas-liquid flow rate ratio. Finally, the volume of generated bubbles in highly viscous liquids in the flow-focusing device is scaled with the capillary number and the gas-liquid flow rate ratio. (C) 2017 Elsevier Ltd. All rights reserved.