Objective: This study was conducted to compare the sealing ability of twocommercially available flowable resin composite materials used for restoration ofClass V cavity preparations. The two compared materials under study were a SelfAdhesive Flowable Resin Composite (Fusio Liquid Dentin) and a ConventionalFlowable Resin Composite (Flow It ALC). Sealing ability was assessed bymicroleakage with and without thermocycling, and at different storage periods of24 hours, three months and six months.Materials and Methods: A total of 120 intact permanent human premolars wereselected for this study. Standardized Class V cavities were prepared on the buccalsurface of the teeth 1.5 mm above the cemento-enamel junction. The teeth wererandomly divided into 2 groups equally according to the type of the restorativematerial used. Half of the restored teeth of each group were thermocycled. Thethermocycled and non-thermocycled restored teeth were stored in distilled wateraccording to the aforementioned storage periods. After the predetermined storageperiods, teeth were coated with nail varnish, and immersed in a 2% Methylene bluesolution for 4 hours, and then longitudinally sectioned in a bucco-lingual direction.Dye penetration was examined with a Binocular Stereomicroscope and scoredseparately for occlusal and cervical microleakage on a 0-3 ordinal scale. Data wereanalyzed with Kruskal-Wallis and Mann-Whitney tests (p = 0.05).Results: Fusio Liquid Dentin had the highest leakage scores, and the differencewas statistically significant (p = 0.001). There were no significant differences forthe effect of thermocycling (p = 0.222), the effect of aging (p = 0.25), or the effectof margin location (p = 0.713) on the leakage scores of the two materials.Conclusion: none of the tested restorative systems achieved a perfect seal, wherethe sealing ability of the self adhesive flowable resin composite underscored that ofthe conventional flowable resin composite. Aging and thermocycling procedureshad no influence on the sealing ability of the two restorative materials; nor did themargin location.