This in-vitro study aimed to evaluate marginal microleakage of CL II cavity preparation restored with three types of resin based dental composite. Forty five sound maxillary first premolar teeth were selected and standardized Class II box only cavities were prepared in the mesial surface of each tooth. The teeth then were restored with micro-filled resin hybrid universal composite (G-ænial POSTERIOR, GC japan). Nano-hybrid universal composite (Filtek™ Z350 XT, 3m ESPE germany). Nanofiber reinforced nano-hybrid universal composite (NovaProTM Fill, nanova usa). After finishing, the teeth were subjected to 500 thermal cycles at 5º-55ºC and mechanical load cycling (50,000 load cycle of 50 N). Teeth were kept in 2% methylene blue for 3 days and then blocked in clear acrylic before sectioning with microtome. Microleakage was evaluated by stereomicroscope (20 X). Data were analyzed statistically by Kruskal-Wallis test. The least microleakage occurred around the group C (NovaProTM Fill) and the maximum microleakage was seen in group B (Filtek™ Z350 XT) but there was no statistically significant difference between the groups.