Purpose: This study was designed to investigate the effect of different flowable composite application techniques and their influence on microleakage of class Ⅱ cavities. Materials and Methods: A Total of 60 human molars were divided into three equal groups according to the way of application and spreading of flowable resin composite by syringe, explorer tip or by ultrasonic tip then each group was subdivided into two equal subgroups according to influence of thermocycling. Cavities were prepared and flowable composite was applied by previous ways then cavities were filled with hybrid composite. Teeth were stored in distilled water, half of specimens were thermocycled then soaked in freshly prepared ammoniacal silver nitrate 2 hours in a dark container then removed and rinsed and immersed in photo developing solution under fluorescence light lamp for 8 hours for evaluation of dye penetration . Teeth were Hemisectioned in a bucco-lingually plane and examined with stereomicroscope.Results: Comparison using ANOVA test showed that the three techniques in regard to non-thermocycling, probe showed highest mean dye penetration while no difference between syringe and ultrasonic tip both showed low results. Thermocycling also showed probe with highest values.Conclusion: Application techniques of flowable composite resin play an important role in microleakage