The process of cutting metal with a plasma arc cutting tool is investigated and discussed. Focus is on the metal cutting process at the inside surface of the kerf. This is an important region that is not well documented due to the difficulty of experiments and the complexity of computation needed to characterize this process. In the present work, a three-dimensional numerical simulation using a plasma model combined with a melting process model is conducted and results are discussed, leading to a better understanding of the physical phenomena within the kerf region of a commercial plasma arc cutting tool. The modeling includes three different phenomena: 1) the plasma jet flow, 2) the Volume of Fluid (VoF) method in identify the gas to molten metal interface, and 3) the phase change model for computing the melting process. The model is implemented in the open source CFD software, OpenFOAM. Thermodynamic and transport properties, calculated by kinetic theory of gases and statistical mechanics, are implemented for accurate simulation in the high temperature regions. The simulation results show the transient cutting process including the physical phenomena for melting of the work piece as well as the plasma flow. The simulated kerf shape is compared to measured kerf under same operations. Additionally, the temperature, velocity, and current density distributions are discussed to understand the plasma characteristics during the cutting process. In an attempt to make a more reasonable kerf shape, the swirl component of the jet, the surface tension and the phase change model are investigated for improvement and discussed. Effects of metal vapor and oxidation reaction are also discussed. This work is a first attempt simulation of the plasma flow, melting, and molten metal flow in the plasma arc cutting process. As the model approaches physical reality, it gives increasingly useful insight into the relationships among operating conditions, providing very helpful directions to improve performance, and providing useful data for designing the plasma arc cutting process.