The influence of the complexity of the molecular structure and the cooling rate in the Avrami model fitting has been studied. Kinetics of non-isothermal crystallization of polypropylenes (with different molecular weights), polyethylenes (High Density Polyethylene, HDPE and Low Density Polyethylene, LDPE) and polyethylene wax from molten state were performed under non-isothermal conditions at cooling rates of 3, 10, and 50 °C min -1 . Differential scanning calorimetry (DSC) was used to monitor the crystallization. The results showed that the predicted evolution of relative crystallinity using the Avrami model was a better fit for the polypropylenes and polyethylene waxen than for the polyethylenes due to their molecular structure much more simple and regular. The fitting for higher cooling rates was better than for lower cooling rates for all the materials analyzed and this could be attributable to a predominant homogeneous nucleation at higher cooling rates and predominant heterogeneous nucleation at lower cooling rates. The fitting for HDPE was better than for LDPE at the final stage of crystallization for various cooling rates due to the phenomenon of a slower secondary crystallization. The results showed a better fitting of the Avrami equation for the polypropylenes and polyethylene waxen than for the polyethylenes due to the influence of the branching degree in lamellar thickness and amorphous-layer thickness as the main contributor to secondary process.