Ultrathin acrylic acid polymer films have been deposited on the surfaces of nanoparticles of ZnO using a plasma polymerization treatment. The average size of nanoparticles is on the order of 50 nm in irregular shapes. High-resolution transmission electron microscopy (HRTEM) experiments showed that an extremely thin film of the acrylic acid layer (15 nm) was uniformly deposited on the surfaces of the nanoparticles. In particular, the particles of all sizes exhibited equally uniform ultrathin films indicating a well-dispersed nanoparticles in the fluidized bed during the plasma treatment. The deposition mechanisms and the effects of plasma treatment parameters are discussed.