The purpose of this study was to evaluate the influence of power density on contraction stress of resin composite restorative materials during photo-polymerization. Six flowable resin composites, and a hybrid resin composite for comparison, were used. The composites were polymerized with the power density adjusted to either 100 or 600 mW/cm2. Stress development was determined with a custom-made tensilometer. The adhesive was placed in a thin layer on a steel rod and resin paste was packed into the mold. The contraction force (N) generated during polymerization was continuously recorded and the maximum contraction stress (MPa) was calculated. Data were analyzed statistically. When the power density was adjusted to 100 mW/cm2, the average contraction stress ranged from 0.30 to 0.50 MPa for the flowable composites, compared with 0.35 MPa for the hybrid composite. When the power density was adjusted to 600 mW/cm2, the average contraction stress ranged from 0.34 to 1.00 MPa for the flowable composites and 0.69 MPa for the hybrid composite comparison. For all materials tested except Estelite Flow Quick, contraction stress increased with higher power density. The present results indicate that contraction stress during polymerization is influenced by power density and resin composite type. (J. Oral Sci. 50, 37-43, 2008)