Nadia M .Zaghloul¹, Naglaa R . Elkholany² ,Basema M . Roshdy^{3 1}Associate Professor of Operative Dentistry, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University. ²Assistant Professor of Operative Dentistry, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University. ³B.Dent,Faculty of Dentistry, Mansoura University. Abstract: Objectives:This study was designed to evaluate the effect of thermocycling on the pattern of microleakage and hybrid layer of two different self-adhesive flowable resin composites in comparison with flowable resin composite with its corresponding two-step self-etching adhesive system. Materials   and Methods:Two class V cavities (3×2×2 mm [width × height × depth]) were prepared on the buccal and lingual surfaces of 42 human molars. Teeth were equally divided into three groups; each group received 14 teeth i.e. 28 cavities, according to the restorative systems used; flowable composite with its corresponding two-step self-etching adhesive system [Clearfil Majesty Flow/Clearfil SE, Kuraray Medical INC., Okayama, Japan] and two self-adhering flowable composites; [Vertise Flow, Kerr Corporation, Orange, CA, USA] and [Fusio Liquid Dentin, Pentron Clinical Technologies, Wallingford, CT, USA]. Each group was further divided into two subgroups, each subgroup received 7 teeth i.e. 14 restorations, according to without or with thermocycling. Five teeth (n=10) from each subgroup were used for microleakage test using 0.15% methanol solution of Leishman dye. The dye penetration was examined under a stereomicroscope and scored. While the other two teeth (n=4) were used for characterization of tooth-restoration interface using Environmental Scanning Electron Microscope (ESEM). Results:Microleakage results of this study showed insignificant differences in the degree of dye penetration between the three restorative systems at either the occlusal or the cervical margin. Thermocycling revealed also insignificant difference in each restorative system. Cervical margins showed insignificant increase in microleakage scores than occlusal margins in each restorative system at either without and with thermocycling subgroup. Despite the insignificant difference, it was found that the flowable composite/ two-step self-etch restorative system (Clearfil Majesty Flow/ Clearfil SE) revealed the least microleakage values, followed by the second self-adhesive flowable composite used in group 3 (Fusio Liquid Dentin). Whereas the first self-adhesive flowable composite used in group 2 (Vertise Flow) showed the highest microleakage values. As for the effect of thermocycling on resin tags length and number, results showed that it had no significant effect on either resin tags length or number. However, the effect of the restorative systems used on resin tags length and number showed significant effect. Results of multiple comparison post hoc Tukeytest of mean values of resin tag length and number were confirmative to microleakage results. Conclusions:Taking into account the sealing ability, the recently developed self-adhesive flowable resin composites are comparative to the flowable composite/ two-step self-etch restorative system. Thermocycling affects the self-adhesive flowable resin composites in the same way as the flowable composite/ two-step self-etch restorative system.  Presence or absence of enamel at the cavity margins is not a matter in the sealing ability of the self-etch adhesive technique.