This paper provides a model of the crystallization kinetics of polyamide 6 (PA6), including primary and secondary crystallization, lamellar thickness distribution, and the evolution of the mobile and rigid fractions of the amorphous phase. The kinetics includes the two-phase structure, the monoclinic α-phase and the pseudo-hexagonal γ-meso phase, of which the fractions depend on the thermal history during solidification. The model is parameterized with experimental results from the literature. The thickness of the rigid amorphous layer was the only parameter to be estimated. The obtained results indicate that the fraction of the amorphous rigid fraction depends not only on the thermal history but also on the crystalline phase and if the rigid amorphous layer was formed in combination with the α- or γ-meso phases. These results provide the bases for predicting and controlling mechanical properties, which can strongly depend on processing conditions as, for example, experienced during injection molding.