The crystallization of Polyamide 12 (PA12) has been investigated using a new experimental setup which allows in-situ synchrotron Wide Angle X-ray Diffraction (WAXD) during Flash-DSC measurements. The experimental results are used to parameterize and validate a new numerical model to quantify the quiescent crystallization kinetics, under isothermal conditions, of the three important crystal structures of PA12, i.e. the, γ− α′− and mesomorphic phase. The experimental approach is based on nucleation and growth after quenching the material from the melt to an isothermal temperature, described by the Schneider rate equations and the Kolmogorov-Avrami expression for the space filling. The experimental overall crystallization rate, expressed in terms of the crystallization half-time, as well as the phase composition, are well captured by the model over a wide range of temperatures, i.e. between the glass transition and the melting temperature. It is shown that at temperature below and above 100°C different nucleation mechanisms are dominant causing the bimodal dependence of the crystallization rate. This work forms the basis for a full model for non-isothermal conditions for which transitions between different phase have to be taken into account.