Abstract: The influence of a lanthanum complex (LaC) on the crystalline characteristics of isotactic polypropylene (iPP) has been investigated using X-ray diffractometer (XRD) and differential scanning calorimeter (DSC). The results of XRD measurements and the melting behavior examined by DSC show that the additive leads to an increase in crystallinity, whereas no obvious change is observed on the crystal modifications. The kinetics of isothermal crystallization of pure iPP (PP) and iPP containing 0.5% of the additive (PP/LaC) have been studied based on the analysis of the DSC experimental data by Avrami equation and Lauritzen-Hoffman theory. The results of Avrami equation analysis reveal that the additive accelerates nucleation and dramatically increases the overall rate of crystallization. The half crystallization period (t1/2) of isothermal crystallization at 130 ℃ decreases from 12.8 min for PP to 7.9 min for PP/LaC. On the other hand, the analysis of Lauritzen-Hoffman theory reveals that the introduction of LaC not only increases nucleus number and accelerates nucleation, but also accelerates the growth rate of crystallization. The nucleation constant (kg) increases from 3.3×105 for PP to 3.8×105 K2 for PP/LaC, while the surface free energy of the developing crystals(σe) decreases from 0.223 J·m-2 to 0.154 J·m-2. All these results suggest that LaC in this system considerably promote crystallization and act as an effective nucleating agent.