In the present paper, the mechanism of droplet formation in immiscible liquid/liquid T-junction system has been studied numerically and analytically. Different droplet stream patterns such as long slugs, small spheres, stable stream connected to a jet and finally parallel flow are observed, which are conventionally called squeezing, dripping and jetting regimes appearing under appropriate numerical setups. We emphasize that the effects of surface force and viscous force in this system alternatively dominate the droplets stream, and the periodically varied pressure and local velocity ensure the generation of uniform droplets. In particular, a two-dimensional lubrication analysis is applied to the confined continuous flow between dispersed emerging phase and the channel wall, which verifies that the pressure buildup upstream facilitate the long slugs formation. The numerical results and theoretical analysis hold a good agreement with the experimental measurements. (C) 2011 Elsevier Ltd. All rights reserved.