A thin film of triphenylaminemethylacrylate was fabricated by physical vapor deposition onto an indium-tin oxide substrate over a temperature range from 230 to 290 K. This thin film was subsequently polymerized by UV irradiation in vacuum with the aim of fabricating electroluminescent devices. Polymerization of the thin film in vacuum was investigated by in situ FT-IR reflection absorption spectroscopy. UV irradiation achieved a polymer conversion of around 100%. Though the polymerization time increased at the lower temperature of the substrate, the surface flatness of the thin film was improved. The polymerization mechanism was elucidated as a simple radical polymerization on the basis of the order of monomer consumption rate. The number-average molecular weight increased with decreasing UV intensity.