Aim: To evaluate the microleakage of three different restorative composite resins through gingival margins of proximal boxes by dye and bacterial penetration techniques. Materials and Methods: Class II slot cavities were prepared for sixty sound human premolar teeth. The teeth were divided into 3 main equal groups of 20 teeth each. Group I cavities were restored with Filtek Z250XT and considered as control ones while those of group II and III were restored with Sonic-filled Filtek Z250XT and Tetric EvoCeram Bulk Fill, respectively. At first, all teeth were undergone thermal cycling and then each group was sub-divided into 2 subgroups of 10 teeth each; according to the microleakage technique used. Results: Monte-Carlo Statistical test showed that the three tested groups had no significant differences among them for the two techniques Conclusions: Sonic-fill technique had the least marginal leakage over the other techniques confirmed by both dye and bacterial leakage testing.