The synergetic effect of dimerized pentaerythritol esters, as novel costabilizer, with metal soap, the mixture of zinc and calcium stearates, as stabilizer, on the undesirable discoloration of poly(vinyl chloride) (PVC) by heating was investigated. The dimerized pentaerythritol esters were prepared by the esterification of pentaerythritol using stearic acid and the dimerization of the obtained pentaerythritol ester using unsaturated dicarboxylic acid. This dimer was used to improve its poor compatibility with PVC. The degree of esterification for pentaerythritol was varied from half to the full amount of hydroxyl groups. Both stabilizer and costabilizer were compounded into poly(vinyl chloride) through roll mixing. As a result, the dimerized pentaerythritol esters showed good dispersibility in poly(vinyl chloride). The effect of suppressing the discoloration was obtained in the systems used, not only the half-esterified type but also the full-esterified type, although it had no hydroxyl group. The abrupt discoloration was caused by the formation of a complex between cool color producing metal chloride, i.e., zinc chloride from the metal soap and double bonds from the dehydrochlorination in poly(vinyl chloride) chain. Usually, the hydroxyl group containing compounds are said to be useful for suppressing the discoloration because the hydroxyl group captures the metal chloride. In the costabilizing effect of the full esterified type, it was clarified from the X-ray photoelectron spectroscopic analysis and infrared spectrum observations for the model mixtures, that the decomposition of the ester group could occur, and the excess zinc chloride was captured as colorless zinc carboxylate.