Aspen (Populus tremuloides) and Nonway spruce (Picea exelsa) wood chips were subjected to 170℃ isothermal laboratory kraft pulping. Hypothetical spherical wood core models were used in the analysis of the rate limiting factor in the kraft pulping process. Under the assumption of a heterogeneous reaction, the carbohydrate dissolution reaction in the kraft pulping process appeared to be limited by the molecular diffusion process. In the kraft delignification process, however, no rate-controlling factor was determined using the heterogeneous model analysis. A volumetric reaction model formulated under the assumption of semi-heterogeneous reaction showed the rate limiting factor in the kraft delignification to be a chemical reaction in both wood species.