A diffusion model was used to analyze mathematically the kinetics for isothermal carburization of pure iron at annealing temperatures between 1011 K and 1185 K. Under such carburization conditions, the austenitic γ phase is produced as a layer on the surface of the ferritic α phase. In the analysis, the diffusion coefficient of carbon in each phase is considered independent of the composition. According to the model, the square of the thickness of the γ phase is proportional to the annealing time. Such a relationship is called the parabolic relationship. The parabolic coefficient is a monotonically increasing function of the annealing temperature and the concentration of carbon on the surface of the γ phase. However, the dependence of the parabolic coefficient on the carbon concentration is less remarkable at higher annealing temperatures than at lower annealing temperatures. The parabolic coefficient varies depending on the annealing temperature in a complicated manner, even if the activity of carbon in the carburization atmosphere remains constant. Thus, the temperature dependence of the parabolic coefficient cannot be described by an Arrhenius equation in the whole annealing temperature range.