The effects of the presence of trapped radicals after UV irradiation were studied on a dimethacrylate oligomer. The postpolymerization reaction was characterized by differential scanning calorimetry and dynamic mechanical analysis. The photopolymerization was realized with 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) as radical photoinitiator by using isothermal photocalorimetry. The postpolymerization was clearly shown in the isothermal mode and under N-2 atmosphere by following the variation of the glass transition temperature of the samples. The influence of crosslinking density, O-2 and thermal postcure were investigated.