Purpose: To evaluate the relation between the linear displacement (LD), shrinkage force (SF) and marginal adaptation of a methacrylate- and a silorane-based composite. Materials and Methods: The LD and SF of 8 samples made of Filtek Supreme XT (methacrylate-based composite) and Filtek Silorane (silorane-based composite) were measured for 180 s from the start of polymerization. Large bulk-filled Class I cavities were restored with both resin composites, and two C-factors were simulated by applying the same adhesive system in different ways: the Silorane System Adhesive (SSA) was applied on enamel and dentin (C-factor 3.5) or only on enamel margins (C-factor of 0.4). Percentages of continuous margins (%CM) were quantitatively assessed with SEM before and after simultaneously loading with 1.2 million mechanical occlusal cycles (49 N; 1.7 Hz) and 3000 thermal cycles (5°C to 50°C) under dentinal fluid simulation. Results: Significantly lower scores of LD and SF were observed for Filtek Silorane (LD: 12.0 µm ± 1.3, SF: 13.7 N ± 1.0) than for Filtek Supreme XT (LD: 25.0 µm ± 0.6, SF: 36.3 N ± 2.9). Both variables, ie, composite type and C-factor, had a significant effect on marginal adaptation (p 0.05). In the groups with high C-factor (SSA was applied on the entire cavity surface) percent of continuous margin (%CM) (mean ± SD) before and after loading, respectively, was 24.4 ± 16.6 and 2.1 ± 2.4 for Filtek Supreme XT, and 58.8 ± 9.9 and 35.4 ± 4.1 for Filtek Silorane. When adhesion was confined to enamel margins (lower C-factor), %CM before and after loading, respectively, increased to 76.1 ± 9.6 and 64.2 ± 11.5 for Filtek Supreme XT, and 96.6 ± 1.7 and 94.2 ± 2.1 for Filtek Silorane. Conclusions: The silorane-based composite exhibited significantly lower shrinkage forces and better marginal adaptation than did the methacrylate-based composite. Keywords: marginal adaptation, silorane, polymerization shrinkage, C-factor