Abstract: Abstract, The performances of heat stabilization of several substituted diphenylureas on PVC were studied by oven discoloration method. It was shown that the substituted diphenylureas were initial- term effective type primary heat stabilizers showing characteristics of heat stabilization similar to zinc stearate and remarkable synergistic effects with calcium stearate and epoxidized soybean oil; the heat stabilizing performance of substituted diphenylureas improved with increasing electron-attracting ability of the groups linked to the urea moiety of the molecules.  Based on this heat stabilizing behavior,  it could he speculated that  substituted diphenylureas stabilized PVC by electrophilic reaction with PVC using their carbonyl carbon atoms as a electrophilic atom.  This meant that the traditional Frye and Horst‘s theory on mechanism of heat stabilization of heat stabilizers did not apply to，substituted diphenylureas，and therefore, the exact mechanism of heat stabilization of heat stabilizers needed to he clarified by further research.

Key words: poly(vinyl chloride);substituted diphenylurea;heat stabilizer, progressive change law;mechanism