It was reported that a nanocrystal ZnO had a thermal stabilizing effect for branched low-density polyethylene (BLDPE). In this study, the thermal stabilizing effect of the nanocrystal ZnO was studied in more detail in comparison to a conventional, micron-size ZnO using several polymers and a common antioxidant. The experimental results in this investigation suggest that the nanocrystal ZnO particles have different surface properties from micron-size particles because of their extremely small size, and the particular surface properties are responsible for the thermal stabilizing effect. (C) 2004 Society of Plastics Engineers.