Aim: This study was performed to assess the effect of types ofcomposite, light intensities, curing modes and thermocycling on toothrestorationinterface. Methods: A total of 160 freshly extracted sound humanpremolars were selected. Teeth were mounted individually in one inchcylindrical acrylic blocks. Specimens were divided into 2 main groups 80 teetheach according to type of resin composite i.e. either using nanofilled resincomposite or using microhybrid resin composite. Each group was furthersubdivided into two subgroup 40 teeth each according to intensity of light i.e.either low intensity or high intensity .Each subgroup was further divided intotwo division 20 teeth each according to curing mode i.e. either soft- startpolymerization or snap shot polymerization .Each division was finallyclassified into two classes 10 teeth each according to thermocycling i.e. eitherdid not subjected to thermocycling or subjected to thermocycling. Standardizedclass v cavities were prepared on buccal surfaces of the teeth. Cavities wererestored with the different two types of resin composite using different lightintensities and different curing modes. Thermocycling was done for a total 500cycles at 5ºC and 55ºC with one minute dwell time at each temperature. Theroots apices were sealed with wax and the teeth were covered with two coats ofnail varnish except 1mm around the restoration then immersed in 0.5 % basicmethylene blue for 24 hours, after rinsing the specimens were longitudinallysectioned and dye penetration was assessed under a stereomicroscope(X 20).Results: whether occlusal leakage or gingival leakage, there was no significantdifference in microleakage of the two types of resin composite when cured withdifferent light intensities of LED (low or high intensity) using different curingmodes ( snapshot or soft start curing modes).there was a significant differencein microleakage between the non-thermocycled and thermocycled specimens.Conclusion: 1-The tightness of seal of resin composites is not affected by thevariation in light intensity of light emitting diode.2-The curing modes of resincomposites have no influential effect on decreasing the marginal leakage ofresin composite.3-Thermocycling has an adverse effect on the marginal sealingof resin composite restorations.