Water-soluble urethane acrylate ionomers containing dimethylolpropionic acid (DMPA) were synthesized, changing the molecular components, and their ultraviolet (UV) coating properties were studied. It was found that the UV coating properties of the urethane acrylate ionomer films were very dependent on the molecular weight of the soft segment, the type of the diisocyanate, and the amount of neutralization. In general observations, the cured films displayed much improved mechanical properties, compared with conventional urethane acrylate film not containing ionic groups. The main reason for the improved film properties seemed to be attributed to the presence of ionic groups in the network. In dynamic mechanical analysis, two distinct glass transition temperatures, corresponding to the ionic hard domains and soft domains, were detected at high content of ionic groups. This suggested that the urethane acrylate network be composed of two phases. Consequently, the ionic hard domains formed by the phase separation from crosslinked network could act as a reinforcing filler, which possibly explains the improved film properties of the urethane acrylate ionomer films.