A penetration process in electroslag welding is simulated using a two-dimensional smoothed particle hydrodynamics method to elucidate the heat transfer mechanism. The base metals are melted and penetration forms in the orthogonal direction of the weld line. From the base metal, molten metal flows downward, and then a molten metal pool forms below a slag bath. The distributions of temperature increasing rates due to heat conduction and Joule heating are also visualized to clarify the dominant factors contributing to the temperature increases of the molten slag and the base metal. Joule heating occurs mainly at the side and bottom of the wire in the slag bath. The temperature of the base metal is increased by the heat conduction from the high temperature slag. The factors of energy transfer in the system are evaluated quantitatively as well. Most of the input electric energy is converted into the thermal energy by Joule heating in the slag bath. Approximately 80% of the generated thermal energy is transported to the base metal, while the other 20% is transported to the molten pool. Compared with the heat transfer in a gas metal arc welding, the thermal energies in the electroslag welding are larger into the base metal and smaller than those in a gas metal arc welding.