Acrylic oligomers having different molecular weights were cured with low molecular weight melamine resin and the influence of molecular weight of acrylic oligomers on accelerated weathering of coating films was discussed. Crack formation time of metallic coating films and gloss retention of white enamel films decreased with decreasing molecular weight of acrylic oligomers. Also the changes in IR spectra and the physical properties, such as glass transition temperature and molecular weight between cross-links, increased with decreasing molecular weight of acrylic oligomers. These results suggest that low molecular weight oligomer system is susceptible to raise the chain reaction during the photodegradation due to the low glass transition temperature of cured films. Change of radical concentration of coating films with UV irradiation time was measured by electron spin resonance method. It was found that radical concentration increased with decreasing oligomer molecular weight, and the linear relationship between the root of radical concentration and crack formation time of metallic coatings was observed.