In order to predict microstructures during vacuum carburizing, the model which simulates not only the carbon(C) diffusion but also growth/dissolution of cementite(θ) is required. For development of a new model we applied vacuum carburizing to low alloy steels and analyzed the distribution of C and θ by GD-OES and image analysis of microstructures. The C in retained austenite(γ) phase after carburizing was also measured by lattice constants obtained from XRD. We also simulated multi-component diffusion with γ matrix and θ layer to analyze a velocity of the moving interface. The new carburizing model was proposed based on the findings, which suggest that C in γ phase at the carburizing surface is supersaturated and corresponds to C concentration for metastable equilibrium condition to graphite. The growth and dissolution of the θ follow a square root of time with the coefficients controlled by diffusion of Si in γ and Cr in θ respectively. The required parameters such as diffusivity coefficients are obtained by the CALPHAD method. The calculated C distributions and volume fractions of θ represent the experimental results.