In the selective laser sintering of polymers, the most widely used powders are based on polyamide 12 (PA12), which is a semi-crystalline polymer. Because the mechanical properties of the printed parts depend largely on the microstructure, knowledge on the crystalline architecture is important. We developed a numerical model based on the finite element method to solve the flow, temperature and crystallization kinetics of PA12 powder during sintering using two different geometries. Our results show that the temperature plays a crucial role in the crystallization kinetics and that simplified 0D calculations can be used to study the crystallization kinetics if the temperature behavior in time at a certain location is known. With our choice of initial and boundary conditions, we found primarily crystals of the α′-phase.