The effect of CaCO3 on the crystallization behaviour and the modification of structure in polypropylene (PP) has been investigated using X-ray diffraction and optical microscopy. The crystallization half time deduced from spherulitic growth rate was found to vary sharply and decreased considerably from 5-6 min for pure polymer to less than 2 min in the presence of CaCO3. The ultimate spherulite size also decreased considerably for PP containing CaCO3. Its dependance on composition however, showed a plateau region at about 10-15 wt % of additive. The intensities of certain reflections especially the 130 and 040 of the α phase were greatly affected by the presence of CaCO3 and large variations in the crystallinity (Ci) values were observed with composition. The ratio of intensities of 130 and 040 reflections and the Ci revealed a maximum at a certain concentration of CaCO3. The above results can be explained on the basis of nucleation and preferential growth of the α phase of PP crystallites. © 1996 Chapman & Hall.