This paper aims at studying the coalescence of bubbles at a microfluidic T-junction convergence by using a high-speed digital camera and the VOF simulation. The microfluidic channels have uniform square cross-section with 400 mu m wide and 400 mu m deep. The responses of bubble collisions at the T-junction convergence have been investigated within a wide range of dimensionless bubble size and capillary number Ca. Colliding coalescence, squeezing coalescence, and non-coalescence were observed at the junction. The result showed that whatever for colliding coalescence or squeezing coalescence, the coalescence efficiency decreases with the increase in the two-phase superficial velocity for moderate liquid viscosities, and the transition from colliding to squeezing coalescence due to the increase in the two-phase superficial velocity enhances the coalescence of bubbles. The decrease in the bubble size for moderate liquid viscosities and the increase in the liquid viscosity are not conducive to bubble coalescence.