Objectives: The current study aimed to evaluate & compare the effect of water storage on marginal adaptation of different flowable resin composite restorative systems bonded to dentin cavities. Materials and Methods: Six different flowable composite restorative systems were used in the current study. A total of 120 human molars was mounted  in a small block of a self cure acrylic resin material, The buccal enamel was removed to expose flat dentin surface. A standardized cylindric class V  cavities were prepared.The cavities were randomly divided into six groups each n=20, according to the composite restorative system used.Marginal adaptation was evaluated immediatelyand after 6 months of water storage, using a metallographic microscope. Results:There was a statistically significant difference between the marginal values of tested groups. No significant difference between the marginal values immediately and delayed of tested groups. Conclusion:Ormocer-based system revealed the bestmarginal adaptation at all testing times.water storage has a negative effect on marginal adaptation of all tested restorative systems.