The structure development and crystallization behaviour of polypropylene (PP) containing nanoparticles of calcium phosphate prepared by a new route based on matrix mediated control of growth and morphology was investigated by X-ray diffraction (XRD), optical microscopy and DSC techniques. The results obtained were compared with that of PP containing conventionally prepared calcium phosphate (CaPO). Both nanoparticle filled PP as well as CaPO filled PP crystallized in monoclinic α crystalline phase. However, considerable changes were observed in the relative intensities of the XRD peaks, especially the peak II (040 reflection), in the case of PP filled with nanosize calcium phosphate. The isothermal crystallization curves showed dramatic increase of crystallization rate and decrease in t1/2 value for PP/nanoparticulate composites resulting in a rapid decrease in the ultimate spherulite size as compared to PP/CaPO samples. The nucleating efficiency was found to be dependent on the particle size as inverse exponential power law. These results were further confirmed by DSC analysis. The optical transparency of the PP/nanoparticle composites was found to be much higher than that containing conventional CaPO for the same concentrations and this could be associated with the reduction in the PP crystallite size. © 2001 Elsevier Science Ltd.