Digital product development process including model-based development（MBD）is being promoted, now on days. In heat treatment process, digital tools are also required. Heat treatment conditions are determined by heat treatment simulation as MBD instead of actual experiments, so simulations that is able to replace experiments are required. In this case, quality assurance of the simulation is important, so we performed quality assurance and improvement for the heat treatment simulation. The heat treatment simulation revealed that the cooling condition on parts surface is important, and it was confirmed that the surface state and the composition of the material to be hardened are important for cooling. In order to improve these, we conducted modeling of the vapor film collapse phenomenon and modeling of the heat transfer coefficient curve. These improvements improve the accuracy of predicting material hardness and deformation. This succeeded in predicting processing quality fluctuations during repeated processing. In order to build analytical models for use in MBD, we will continue to develop methods to easily check quality by separating important and non-important parameters and building verification models for important parameters.