Boiling heat transfer in vertical rectangular mini-channels with a width of 1mm and 0.1mm is studied in this paper. The course of bubble generation, growth and departure is numerically simulated and analyzed, and the influence of the movement of phase interfaces on pressure difference and average surface heat transfer coefficient is investigated by using geometry reconstruction and interface tracking. All the calculation is conducted taken into account the gravity, surface tension and wall adhesion. This paper presents that the width of channels plays a significant role in the course of bubble generation, growth and motion, which also leads to changes of critical heat flux. In addition, it is found that surface tension is much more important than gravity in the process of boiling heat transfer. As the channel size is decreased, the boiling heat transfer coefficient increases significantly, which proves that mini-channels can enhance the heat transfer. However, the boiling heat transfer coefficient obtained through numerical simulation is higher than the existing experimental data due to the ideal assumptions adopted in simulation.