A three dimensional numerical simulation on the droplet formation in a microfluidic T-junction is reported. The mechanism of droplet formation in the immiscible liquid/liquid T-junction system has been studied numerically with the Level Set Method (LSM). Various droplet flow patterns such as long slugs, small spheres, stable stream and finally parallel flow are investigated, which are conventionally called squeezing, dripping and jetting regimes correspondingly. Apart from those, the transition regime between squeezing and dripping as well as parallel flow regime are also discussed. The pressure of the junction point fluctuated in the droplet formation process and it can reflect the flow pattern in the microchannel exactly and directly. The simulation results show that the pressure profiles of the junction point in the squeezing, dripping and jetting regimes change during the droplet formation process. New insights on the pressure of the junction point during the droplet formation process are provided and the results show that the frequency of the pressure fluctuation is the same as that of new droplet formation. In addition, the pressure of the junction of different contact angles of the dispersed phase with the surface and different capillary numbers (Ca) are also analyzed for investigating the droplet formation mechanism. The amplitudes and periods of the pressure in different regimes differ from each other. At the same flow velocity, the amplitudes of the pressure in dripping regime, squeezing regime, jetting regime and parallel flow regime ranks in the descending order. (C) 2012 Elsevier Ltd. All rights reserved.