Objectives: To evaluate the marginal adaptation and microleakage of the enamel and dentin of direct classII resin composite restorations. Materials and methods: Forty standardized class II MOD cavities were prepared with one gingival margin occlusal to cementoenamel (CEJ) margin and the other one extending apical to CEJ, then teeth were assigned into four equal groups (n=10) to be restored with four one-step self-etch adhesives: Adper Prompt L Pop (pH 0.9-1), Tetric N Bond (pH 1.7), Clearfil S3 (pH 2.3), and Bond-1 SF (pH 3~4). The cavities were then restored with an incrementally placed nanohybrid resin composite (Tetric EvoCeram), before they were stored in distilled water (48 hours, 37ºC). Teeth were thermocycled (2000 cycles, 5ºC to 55ºC), and mechanically loaded (50,000 cycles, 50 N). Impressions of the teeth were taken and epoxy resin replica were made. The replicas were gold sputtered and the gingival margins were scanned using quantitative SEM analysis. Additionally, The teeth were immersed in 0.5% basic fuchsin solution for 16h at 37ºC, and sectioned mesiodistally to obtain specimens for microleakage testing. Data were analyzed with Kruskal Wallis and Mann Whitney tests (P ≤ 0.05). Results: There was no significant difference between the four adhesives in the SEM analysis at both enamel and dentin margins. Tetric N Bond showed significantly higher dye penetration than the other three adhesives at the dentin margins. Bond-1 SF showed significantly lower dye penetration than the other three adhesives at the enamel margins. Conclusions: Based on the results of the current study, the tested adhesives seems to provide clinically relevant marginal seal in class II resin composite restorations regardless the pH value.