Objectives: The objective of the present study is to evaluate and compare the marginal adaptation of one bioactive bulk fill material and three different bulk-fill resin based composites. Materials and methods: Class II slot cavities were prepared in eighty sound human molar teeth. The teeth were divided into four equal groups (n=20). Group I: cavities were restored with Activa bioactive restorative, Group II: cavities were restored with Sonic-fill bulk fill, Group III: cavities were restored with Tetric EvoCeram Bulk Fill and Group VI: cavities were restored with Filtek Posterior bulk fill respectively. All teeth were subjected to thermocycling, gold sputtering and marginal adaptation was evaluated using scanning electron microscope. Collected data were analyzed by kurskal Wallis test. Results: No significant differences in marginal adaptation were found between the four tested groups (P< 0.05). Conclusions: Under the limitations of this in vitro study, it was concluded that all the tested bulk fill materials showed acceptable marginal adaptation after thermocycling with no significant differences between them.