Abstract Vacuum carburizing is a method to strengthen the surface of low quality metals. Metal like raw steel is carburized by reactive gas (e.g. acetylene or propane) at low pressure (below the atmospheric pressure) and high temperature (above 1000 K) in a furnace. Carburized metal is roughly 1.5 times harder than raw metal. The carburized metals are used for industrial application to prevent wear-out of the metals under rotating and contact situation, for example, gear of gas turbine. In this paper, numerical modeling of vacuum carburizing by acetylene gas diluted by argon gas is described. Gas phase and solid phase are taken both into account as fluid–solid interaction in the vacuum carburizing process. In the numerical method, simple reaction mechanism of surface reaction of carburizing on the steel is assumed. Carbon transfer is also calculated. Commercial computational fluid dynamics software ANSYS FLUENT 13 with user defined function is utilized for the calculation. The aim of this study is to show how well our proposed approach can describe physical phenomena, and to be capable of finding optimized operation of vacuum carburizing by our proposing numerical simulation. Experimental study is conducted using a heat chamber. Acetylene/argon gas mixture is supplied in quartz pipe heated by the chamber. Quadrupole mass filter is used to measure acetylene, hydrogen and argon partial pressure. Carburized carbon mass weight is measured by electronic balance before and after carburizing. Furthermore, local carbon mass fraction is also measured by Electron Probe Micro Analyzer (EPMA). Metallurgical microscope, which is to see metallic microstructure, is also utilized to investigate surface of the carburized steel. The numerical results give mole fraction of hydrogen and acetylene in the gas, carburized carbon mass weight and distribution of carbon mass fraction inside the steel. Our numerical results are in good agreement with experimental results. It indicates that Computational Fluid Dynamics can be extended to the application in carburizing processes, and be a useful tool for improving carburizing processes.