The crystallization of a polymer melt, taking place during transformation processes, has a great impact on the process itself, mainly because it causes a large increase in the viscosity (hardening). Knowledge of the hardening kinetics is important for modeling and controlling the transformation processes. In this work, first an overview is given of the experimental and modeling work on the hardening of crystallizing polymers. Next, we present isothermal crystallization experiments using differential scanning calorimetry (DSC) and rotational rheometry to measure the dynamic viscosity. The evolution of the relative crystallinity and normalized complex viscosity are correlated by a novel technique which allows simultaneous analysis of several runs, even if they are not carried out at same temperatures; the main requirement with the traditional technique. The technique, described in detail in this paper, provides an experimental relationship between the crystallinity and the hardening, i.e. the increase in the viscosity. Moreover, by measuring the dynamic viscosity at different frequencies, surprisingly, a master curve is obtained which combines the effects of shear rate, temperature and the level of crystallinity.