This paper describes a microemulsion-mediated synthesis of zinc oxide nanoparticles for varistor studies and possible applications. These ultrafine polycrystalline ZnO nanoparticles have size ranging from 5-40 nm ESD. The microstructure, high density and electrical conductivity of sintered disks of pure and doped ZnO nanoparticles resulting from clean interfaces of particles and scaled down size of grains are found to result in a higher critical electric field and a higher coefficient of nonlinearity (alpha) in the logE versus logJ curve. Significant developments will rely on further understanding of structure-property relationships.