Background: This study compared in vitro the marginal adaptation of three different, low shrink, direct posteriorcomposites Filtek™ P60 (packable composite), Filtek™ P90 (Silorane-based composite) and Sonic fill™ (nanohybridcomposite) at three different composite/enamel interface regions (occlusal, proximal and gingival regions) of astandardized Class II MO cavity after thermal changes and mechanical load cycling by scanning electronmicroscopy.Materials and methods:Thirty six sound human maxillary first premolars of approximately comparable sizes weredivided into three main groups of (12 teeth) in each according to the type of restorative material that was used:group (A) the teeth were restored with Filtek™ P60 and single bond™ Universal adhesive using horizontal incrementaltechnique, group (B)the teeth were restored with Filtek™ P90 and P90 system adhesive using horizontal incrementaltechnique and group (C) the teeth were restored with Sonic fill™ composite and single bond™ Universal adhesiveusing bulk technique.After specimens were stored in distilled water at 37°C for 7 days, all specimens were subjectedto thermocycling at (5° to 55 °C), then submitted to mechanical load cycling (intermittent axial force of 49N and atotal of 50.000 cycles). The specimens were observed under scanning electron microscope at (2000 X) to measuremarginal gap width (the distance between the dental wall and the restoration) at occlusal, proximal and gingivalregions in micrometer using Tescan software, version 3.5. Data were analyzed statistically by one way ANOVA testand least significant difference tests.Results:The results showed that the silorane-based posterior composite (Filtek™ P90) showed significantly the leastmarginal gap width at the occlusal, proximal and gingival regions after the application of thermal changes andmechanical load cycling in comparison to the two methacrylate-based posterior composite Filtek™ P60 (packable)and the Sonic fill™ (nano-hybrid). Sonic fill™ bulk fill composite that relied on the vibration concept to lower theviscosity of high filler loaded composite material showed significantly lesser marginal gaps width at occlusal, proximaland gingival composite/enamel interface regions in comparison with Filtek™ P60 (packable composite) usinghorizontal incremental technique. The silorane-based composite (Filtek™ P90) showed non-significant difference inmarginal gaps width at the three different regions. While, both methacrylate based Filtek™ P60 and Sonic fill™composite showed significantly lesser marginal gap width at the occlusal region in comparison with gingival regions.Conclusion: None of the low-shrinkage composite restorative materials tested in this study totally prevented microgapformation at composite/enamel interfaces of Class II MO cavity.