An in situ deposition approach was used for the synthesis of nano-CaSO4 and nano-Ca-3(PO4)(2). The nano-size particles were confirmed with an X-ray diffraction technique. Composites of polypropylene (PP) with 0.1-0.5 wt % nano- or commercial CaSO4 or nano-Ca-3(PO4)(2) were prepared. The transition from the alpha phase to the beta phase was observed for 0.1-0.3 wt % nano-CaSO4/PP and nano-Ca-3(PO4)(2)/PP composites. This was confirmed by Fourier transform infrared. A differential scanning calorimetry analysis was carried out to determine the thermal behavior of the nanocomposites with increasing amounts of the nano-CaSO4 and nano-Ca-3(PO4)(2) fillers. Increases in the tensile strength and Young's modulus were observed up to certain loading and were followed by a decrease in the tensile strength. A continuous decrease in the elongation at break (%) was also observed for commercial CaSO4 and larger nano-Ca-3(PO4)(2). A decrease in the mechanical properties after a certain loading might have been due to the agglomeration and phase transition of PP in the composites. (c) 2006 Wiley Periodicals, Inc.