Purpose: The effect of the slow-start curing method on the marginal sealing and cavity wall adaptation on resin composite restorations with different C-factors was evaluated. Methods: Cylindrical cavities, 1 mm deep and 3 mm in diameter (C-factor = 2.3) or 2 mm in diameter (C-factor = 3) were prepared on flat superficial bovine dentin surfaces. The teeth were restored with Clearfil Photo Bond, Clearfil Liner Bond 2 or Super-Bond D Liner adhesive systems followed by Photo Clearfil Bright composite. The resins were cured with a conventional method using 600 mW/cm² (tip-to-resin distance 0 mm) for 60 s, or the slow-start curing method of 270 mW/cm² (tip-to-resin distance 10 mm) for 10 s, followed by a 5-s interval, then 50 s at 600 mW/cm². After thermocycling, a dye penetration test was carried out. The dye penetration length was calculated as a percentage of the total cavity wall length. Results: Cavity-wall gap formation increased when the C-factor increased from 2.3 to 3, except in one material, Super-Bond D Liner, which showed good marginal sealing and resin composite adaptation to the cavity wall regardless of light curing method and C-factor. Conclusion: It is necessary to take care when a cavity with a high C-factor is to be restored with resin composite. The combination of a flexible adhesive and the slow-start curing method would seem to be effective in reducing polymerization contraction stress for large C-factor cavities.