OBJECTIVE The objective of this study was to determine the distribution and the magnitude of the internal stresses in a resin composite restoration resulting from polymerization shrinkage by using photoelastic analysis. METHODS Butt-joint box-shaped cavities (5.0 x 2.0 mm, 2.0 mm in depth) prepared in bovine teeth and in composite moulds were filled with the light-activated transparent composite. The restoration was cross-sectioned perpendicularly to the longitudinal side of the cavity and observed with polarized microscopes. The principal stresses of the restoration, normal and shear stresses at the cavity wall were evaluated by the graphical integration method. The integrity of the bond along the cavity wall was also examined by staining method. RESULTS The internal stresses of the restorations in bovine teeth were not large enough to observe, apparently because the gaps along the dentinal wall acted as a stress relief. On the other hand, there were no gaps along the cavity walls at the restorations in the composite moulds. As flow of the resin composite was severely limited, the maximum normal tensile stress at the cavity wall, which occurred near the internal line angle of the cavity, was calculated to reach as high as 23 MPa. The stress level near the internal line angle was higher than that near the cavo-surface margin. CONCLUSIONS The distribution of the internal stresses in a composite restoration in a box-shaped cavity is considered to be unfavorable for the deep dentin bond. A good understanding of these phenomena may improve the clinical effectiveness of resin composite restoration.