Abstract: The heat stabilization of several N-(substituted phenyl)-N′-cyanoacetylureas on PVC was studied by oven discoloration method.It was found that the N-(substituted phenyl)-N′-cyanoacetylureas gave rise to an initialstage heat stabilization and a remarkable synergitic effect with calcium stearate and epoxidized soybean oil.The heat stabilizing performance of N-(substituted phenyl) -N′-cyanoacetylureas improved with increasing electrondrawing ability of the group linked to the urea’s imino group,based on which a progressive change law of heat stabilizing was deduced that the N-(substituted phenyl)-N′-cyanoacetylureas stabilized PVC by electrophilic reaction with PVC using their carbonyl carbon atoms as an electrophilic atom.This meant that the traditional Frye and Horst’s theory on mechanism of heat stabilization of heat stabilizers did not apply to N-(substituted phenyl)-N′-cyanoacetylureas,and therefore,the exact mechanism of heat stabilization needs to be clarified by further research.