A quartz crystal microbalance (QCM) technique which can detect weight increase in the nanogram order has been employed to continuously monitor the iniferter-based surface photograft polymerization behavior of vinyl monomers. The gold electrodes of the QCM were thinly coated with a photoreactive polystyrene partially derivatized with N,N-diethyldithiocarbamylmethyl groups acting as iniferters. A monotonic decrease in the resonance frequency of the vibrating oscillator of QCM, which was correlated with the weight increase via graft copolymerization initiated on the polystyrene, occurred immediately after ultraviolet light irradiation of the coated electrode surface in a methanolic or aqueous solution of vinyl monomers such as acrylamide (AAm), N,N-dimethylacrylamide (DMAAm), methacryl amide (MAm), methacrylic acid (MA), ethyl acrylate (EA), 2-hydroxyethyl methacrylate (HEMA), 2-(dimethylamino)ethyl methacrylate (DMAEM), N-vinyl-2-pyrrolidone (VP), and styrene (ST), except for vinyl acetate (VA). Upon cessation of irradiation, little frequency change was observed. The cycle comprising a decrease in resonance frequency under irradiation and no change under dark condition was repeated without appreciable fatigue. The polymer yield showed an almost linear increase with irradiation time up to 20 min. The apparent polymerization rate increased with concentration of monomer, light intensity, and surface density of dithiocarbamate groups. In addition, the rate was dependent on monomer＇s Q value and maximum at Q values of around 0.7-0.8 for EMA and HEMA.