In order to understand the dechlorination mechanism of polyvinyl chloride (PVC) by its grinding with Ca(OH)(2), computational simulation on the chemical interaction between 4-carbon vinyl chloride (4C-VC) as a model of PVC and Ca(OH)(2) has been performed by semi-empirical and ab initio methods. The lowest binding energy in the 4C-VC model molecule is the C-Cl bonds, and chlorine IS ccompanied by hydrogen when it is removed from the molecule by external force. When the Ca(OH)(2) molecule approaches the 4C-VC, the potential energy difference between the two molecules in the excited state IS lways lower than that of the grand state, and the excited system is stable at 0.33 mm Ca-CI distance. When the distance between the two molecules is kept constant, the excited state has three minimum potential energy differences at around 0.22, 0.27 and 0.33 nm C-Cl distance, while the grand state has only one minimum potential energy difference at 0.22 nm. This implies that the 4C-VC molecule is stabilized by joining with Ca(OH)(2) even after breakage of the bond by an external force. Ca(OH)(2) plays an important role as a receiver of HCl from the 4C-VC molecule. This is well consistent with the experimental results from mechanochemical dechlorination of PVC by its grinding with Ca(OH)(2), forming hydrocarbon and CaOHCl.