Crystallization behaviour of i-polypropylene in non-isothermal conditions was studied by means of WAXD using monochromatised X-ray synchrotron radiation. It is demonstrated that the crystallization rate during continuous cooling with a constant rate is not only a function of actual, momentary temperature, but also depends on the cooling rate. Near the melting temperature, the measured values of non-isothermal crystallization rate are close to those evaluated from isothermal crystallization, while at lower temperatures the difference between both quantities increases substantially. Interpretation of the results, based on Ziabicki's model of crystallization kinetics, offers a new approach leading to the estimation of relaxation time characterising the transient course of crystallization.