The effect of configuration factor (C-factor) on cavity adaptation was investigated in three compomer and one resin composite restorations. Eighty-four cylindrical dentin cavities (C-factor: approximately 2.5, 3.0 or 4.0) prepared on flat coronal dentin surfaces were filled with the materials in combination with their proprietary adhesive systems. Cavity adaptation was microscopically examined after 15 minutes storage in water at the top surface and at other four sites along the cavity walls. Additionally, indentation testing was performed for each material at 20 minutes and 24 hours after irradiation. Regression analysis revealed no relationship between C-factor and gap dimension in compomer restorations at any of the measuring sites, while a logarithmic relation was found only at the cavity floor of the composite fillings. All materials showed maturation of mechanical properties. The elastic component of the indentation was smaller in compomers than in the composite. It was concluded that C-factor had no influence on the cavity adaptation for compomer restorations. This might be due to reduced stress generation at the bonding interface caused by relatively low mechanical properties immediately after curing, less elasticity, and water absorption in compomers.