Fundamental mechanisms of selective laser melting (SLM) of metal powders in additive manufacturing (AM) were investigated numerically and experimentally. A simplified 2D finite element model of multiphase fields was proposed to simulate the SLM process based on the conservation equations of mass, momentum and energy. Multiple dynamic physics/ phenomena considered in this work include heat transfer, solid/liquid and liquid/vapor phase changes, vapor pressure, surface tension, gravity, melt flow, gas flow, wetting and bonding of powder particles with the melt, and re-solidification. To deposit laser energy to the powder bed, the liquid/gas interface was tracked using a level set method. The numerical simulation was carried out using COMSOL Multyphysics®. To validate the proposed methodologies, an SLM experiment was performed for Ti6Al4V powders. It was shown that the simulation results of the cross-section shapes and the heights of re-solidified parts are in good agreement with the experimental measurements.