A model is proposed in this work for progressive nucleation to interpret the measured metastable zone width (MSZW) by the polythermal method using the classical nucleation theory. As similar to Kubota's model Ii I, the MSZW limit is assumed to correspond to a point at which the number density of accumulated crystals has reached a fixed (but unknown) value during the cooling process. The model yields an integral formula to directly determine the interfacial energy of the crystallized substance from the measured MSZW data at various cooling rates, It is found that the model fits well with the experimental data in the literature. (C) 2014 Elsevier B.V. All rights reserved,