Formation of satellite droplets during drop formation or breakup leads to an increment of polydispersity of emulsion and runs contrary to the original intention of microfluidics. Thus, more attention is needed to shed light on the mechanism of satellite droplets formation to alleviate polydispersity. In this work, we studied the satellite droplet formation process in microfluidic flow-focusing junction and T-junction with capillary number Ca = mu(c)u(c)/sigma in a range of 0.039-0.32. The formation of satellite droplets during the thinning of neck filament was recorded by high speed camera. Using self-written programs, the evolution of the minimal width, the volume of and the length of neck filament were analyzed to understand the satellite formation process. It was found that before final pinch-off, liquid neck expriences four different thinning regimes. There is a critical length L-c = mu(c)mu(d)/sigma rho(d) during the evolution of the neck thinning processes. When the neck width is less than the critical length, the thinning processes always experience the transition from rapid shrinking to slow shrinking facilitating the formation of satellite droplets. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.