Polyropylene (PP)- zinc oxide (ZnO) nanocomposites were obtained by a melt mixing process. Organo silane-modified ZnO and untreated ZnO, and compositions with added stearic acid were prepared in a twin screw extruder. Thermal properties of the composites were determined by differential scanning calorimetry. The obtained composites were foamed by conventional injection molding using the chemical foaming agent azodicarbonamide. The effects of the ZnO addition and the coupling agent on cell morphology, tensile strength, and weight reduction were investiagted. The silane-treated ZnO was effective in cell generation; however, this decreased the tensile strength of the composite foams. On the other hand, untreated ZnO induced a slight increment in tensile strength that was due to lower cell generation. When weight reduction and improvement in tensile strength were considered togehther, the addition of 3 wt. % of untreated ZnO was found to be successful for PP foaming.