Abstract：This article describes the preparation of PVC/nanometer CaCO3 composite particles by an in situ method. Porous polyvinyl chloride (PVC) particles were swelled by compound solvent with foaming agent 2,2￠-azo- bis-iso-butyronitrile (AIBN). When heated at 112oC, PVC particles were foamed because AIBN decomposed to gas. When the volume percentage of cyclohexanone in compound solvent reaches 11.7%, the porosity of microporous PVC increases to the maximum of 35.9%. Microporous PVC, Ca(OH)2 and some additives were dispersed uniformly in water and then aerated with CO2, thus PVC/nano-CaCO3 composite particles was prepared by this in situ method. Chemical analysis indicated that the maximal content of synthesized CaCO3 reached 20%. SEM micrographs show that microporous PVC particles had many cavities and nanometer CaCO3 were filled in these cavities at the same time. It is inferred that nanometer CaCO3/PVC mixture toughens and reinforces PVC.