Statement of Problem: There is a principle rule about marginal seal in durability and clinical success of restorative treatments. Polymerization shrinkage and microleakage are critical limitations of dental composites.Purpose: The aim of this study was to evaluate the effect of two different curing units on microleakage of class V composite restorations in different conditions.Materials and Method:  In this in vitro study, class V cavities were prepared on the buccal surfaces of 80 extracted premolar teeth. Conventional halogen curing unit (optilux 501) and high intensity light-emitting-diodes (LEDmetron) were used to polymerize resin composite (Z100) from two distance 2 and 8 mm. Four groups after 24 hours and the other four groups were restored for 3 months in water at 37oC, immersed in dye solution and then teeth were sectioned longitudinally and evaluated for dye penetration. Data were statistically analyzed using Kruskal-Wallis and Mann-Withney tests to compare the different groups. Results: Using Kruskal Wallis test, no significant difference in microleakage was demonstrated between the different light cure units in the 8 groups. Mann-Whitney test at the dentin side showed that, QTH could significantly reduce the microleakage compared to LED (p=0.007). Increase in distance and storage time did not influence the microleakage. At 2 mm distance, the difference between LED and QTH was significant (p=0.038), but no significant difference was seen at 8 mm distance. Increased distance for LED and QTH did not cause any microleakage.Conclusion: According to the results of this study, it can be concluded that the use of turbo light guide in LED would decreases the output of the light. Therefore, QTH system is recommended to be used in shallow cavities. Key words: Light cure units, Microleakage, Distance, Storage time