The kinetic crystallization model of Avrami is generally accepted as a starting point for the analysis of isothermal polymer crystallization. It is shown in this paper that, in the case of non-isothermal crystallization kinetics with constant heating or cooling rates, an apparent m-order reaction model is approximately equivalent to the nucleation and growth model. of Avrami in the vicinity of the inflection points of the corresponding crystallization curves. Since the apparent m-order reaction model is defined for every real, positive apparent reaction-order m, a distinct Avrami index n, which is valid for the characterization of isothermal and nonisothermal crystallization experiments with constant heating or cooling rates, can always be related to any apparent reaction-order m. Therefore, two types of Avrami master curves, which are dependent merely on the Avrami index n and which describe the isothermal polymer crystallization thoroughly, can be obtained by performing non-isothermal experiments with constant heating or cooling rates.