Semi-crystalline polymers typically consist of crystalline and amorphous domain with dimension in the nanometer range. The final properties of such material depend on the volume fraction of the crystalline domains as well as on their size and structure. Polyamide 12, as well as many others polymer, shows a bimodal dependence of the crystallization rate as function of crystallization temperature. Depending on the processing conditions PA12 shows a complex polymorphism: ’, , α, α’. The main crystal form obtained by slow cooling from the melt at room temperature is the  form. The ’-form can be obtained by fast quenching from the melt and it transforms in to -form upon annealing. The α-phase is obtained only under specific conditions such as high pressure crystallization, casting from solution and drawing close to the melting point. The α’-form has been observed for crystallization at high temperatures. In order to study the isothermal crystallization of semicrystalline polymer in the whole temperature range between the glass transition temperature and the melting temperature, high cooling and heating rates are required to be sure that the crystallization takes place isothermally. In this research Flash DSC experiments were combined with in-situ wide angle X-ray scattering at high time resolution. We found that high super-cooling from the melt leads to only one strong reflection characteristic of the hexagonal structure of the ’-form and -form. At lower supercooling, PA12 crystallizes into α’-form showing the two reflection peaks characteristic of a monoclinic structure. Most of these transformations are reversible during the heating and cooling, and thus we are able to investigate the structural changes of PA12 with in-situ X-ray experiment during fast heating/cooling. These results will provide insight in the complex phase behaviour of PA12 and the obtained data is used to develop a full crystallization model, based on well established knowledge present in our group, that will permit to predict the final structure of the material depending on the processing conditions.