Calcium carbonate was synthesized by in situ deposition technique and its nano size (35-60 nm) was confirmed by transmission electron microscopy (TEM). Composites of the filler CaCO3 (micro and nano) and the matrix poly(vinyl chloride) (PVC) were prepared with different filler loadings (0-5 wt%) by melt intercalation. Brabender torque rheometer equipped with an internal mixer has been used for preparation of formulations for composites. The effect of filler content both nano-and micro level on the nanostructure and properties is reported here. The nanostructures were studied by wide angle X-ray diffraction and scanning electron microscopy. The mechanical, thermal, and dynamic mechanical properties of PVC/micro- and nano-CaCO3 composites were characterized using universal testing machine, thermogravimetric analyzer, and dynamic mechanical analyzer. The results of thermal analysis indicated that the thermal stability of PVC/nano-CaCO3 composites was improved as compared with corresponding microcomposites, and that of pristine PVC and maximum improvement was obtained at 1 and 3 phr loadings. However, the tensile strength decreased significantly with increase loading of both nano- and micro-CaCO3, whereas storage modulus and glass transition temperature increased significantly.