Nano ZnO particle was synthesized by precipitation method using Zinc sulfate (ZnSO4.0.5H2O) as a precursor, and sodium hydroxide solution was used as a medium with and without the dispersant polyacrylamide. Various factors affecting the preparation such as pH, temperature, and crystallization time, were investigated. The nano ZnO particles were characterized by X-ray diffraction and SEM. It was found that nano ZnO particles with the size about 15 nm was successfully produced with high crystallinity and purity. The optimum conditions for nano ZnO preparation were pH 9, the temperature at 40oC, and crystallization time of 1 hour. The synthesized nano ZnO particle as an activator for sulfur vulcanization of rubber was studied. It showed that the synthesized nano ZnO has activated rubber vulcanization process faster and created more cross-linking.