The aim of this study to evaluate the effect of Er,Cr:YSGG laser versus conventional bur on marginal integrity of resin composite for cavity preparation. 40 extracted human permanent molars were used in this study and were divided into two main equal groups according to cavity preparation technique (class V cavities prepared by conventional diamond bur and Er,Cr:YSGG laser). Each group was divided into two subgroups according to subjection to load cycling. Each subgroup was further sub divided into two classes according to aging period (7 days and 3 months). Scotch bond universal adhesive system was applied to all cavities according to the manufacturers instruction and cured for 20 seconds, then restored with composite resin filtek Z350 and light cured for 40 seconds. 2000 cycles were applied at loads between a minimum 10 N and a maximum 89 N force with a load profile in the form of a sine wave at frequency of 1 HZ.Samples were aged in distilled water according to the subdivision (7 days & 3 months). Data were analyzed with Kruskal-Wallis and Mann-Whitney U tests after methylene blue dye immersion for 24 hours and mesiodistal sectioning through the center of the restoration. Results of this study will be summarized in statistically non- significant difference between conventional and laser cavity groups in the microleakage scores.