Polymer nano-composites, filled with nano-particulate fillers, are alternatives to the conventional filled polymers. In this study, poly (vinyl chloride)/nanometer calcium carbonate (PVC/nano-CaCO3) composite resin was prepared via in-situ suspension polymerization in the presence of treated and untreated nano-CaCO3. In situ polymerization was caried out by two methods. In the first, nano-fillers together with initiator are pre-dispersed in VCM and then aqueous phase containing suspending agents is charged into the reactor.in the second serious of experiments, first a dispersion of nano fillers in water and suspending agents is made by ultrasonication and then charged into the reactor containing water and initiator. The grains produced by both methods were analysed by SEMand EDX tests. The elemental analysis obtained from EDX test show that PVC grains produced from the first method have a higher content of nano-particles compared to those prepared by second one. The K-value of the grains prepared by first method decreases with nano-filler content whereas it remains unchanged for those obtained by second one. As the interfacial tension of the dispersion system consisting of CaCO 3 is shifted towards larger value, the agitation speed and the amount of suspending agents was set higher compared to which carried out in the absence of nano-fillers to obtain a stable system. The results show that level of conversion and the time of reaction does not change with addition of nano-particles. The SEM pictures of PVC grains indicate that the in situ polymerization causes more regular, spherical shape particles. Also, SEM images from sectioned PVC grains show that the primary particles tend to be smaller in size with increasing nano-filler content, thereby resulting in more pores particles and subsequently increasing in CPA. PVC grains by second method, CPA decreases and corresponding BD increases with increasing filler content. The mechanical properties characterized by tensile strength, modulus and impact strength increases and, as expected, elongation at break decrease with nano-particle content. Outstanding result was the increase of impact strength by 2.5 times for product prepared by first method compared to the sample is produced by pure PVC. Key Words: Nanocomposite, In situ suspension polymerization, nano-CaCO 3 ,
