Dynamics for droplet breakup with tunnels and formation of satellite droplets in a symmetric microfluidic T-junction are investigated using a high-speed digital camera. The breakup process of droplet could be divided into four sequential stages: squeezing, transition, pinch-off, and thread rupture stages. Effects of the viscosity ratio of both phases lambda, the capillary number of the continuous phase Ca-c, and the dimensionless droplet length l(0)/w(c) on the breakup process of droplet are analyzed. In the squeezing stage controlled by squeezing pressure, the variation of the minimum width of the droplet neck with time could be scaled as a power-law relationship with exponent related to lambda and l(0)/w(c). The transition stage is dominated by the velocity of the fluid, and the pinch-off stage is mainly driven by the capillary force. For these two stages, the evolution of the minimum width of the droplet neck with the remaining time could be described as other power-law relationship with exponent related to lambda. In the thread rupture stage controlled by viscous stresses of both phases and the surface tension, the minimum width of droplet neck decreases linearly with time, and the coefficient of the linear function depends on lambda. Additionally, for the formation of satellite droplets, the size of satellite droplet increases with the increase of superficial velocity of the fluid and droplet viscosity. Furthermore, a critical capillary number 0.03 is observed in low viscosity of the droplet, which divides the variation of the size of the main satellite droplet with the capillary number into constant area and growth zone. (C) 2018 Elsevier Ltd. All rights reserved.