Daptation of two types of bulk fill resin composites; ormocer-based (Admira Fusion X-tra) and non ormocer-based (X-tra fil) used to restore occluso-mesial and occluso-distal cavities using 3 different restorative techniques (bulk-fill, split-bulk, and centripetal techniques).Materials and Methods:A total number of 48 freshly extracted human molars were selected. The teeth with mesio-distal and bucco-lingual dimensions of (10-11 mm and 5-6 mm) respectively were included in this study. The selected teeth were divided into two equal groups 24 teeth each according to the type of resin composite used (A); ormocer-based bulk fill resin composite, Admira fusion x-tra (A1) and non ormocer-based bulk fill resin composite, X-tra fil (A2). Each group was then sub-divided into three sub-groups 8 teeth each according to the application technique of resin composite (T); bulk fill technique (T1), split-bulk technique (T2) and centripetal technique (T3).Standardized occluso-mesial and occluso-distal cavities were prepared in each tooth such that 2.5 mm of the remaining occlusal tooth structure was preserved between the both cavities. The prepared cavities were restored using one type of etch and rinse adhesive system (Solobond M) followed by application of bulk fill resin composite, either Admira fusion xtra or X-tra fil resin composite.The bulk fill resin composites were applied using one of three restorative techniques; the bulk technique where resin composites were applied in bulk (4 mm increment) according to manufacturers' instructions. The second technique termed split-bulk technique where resin composite was applied in single increment then splitted in a horizontal direction intotwo parts to create an empty split which was further filled with single increment of resin composite. The third was the centripetal technique, in which an increment of resin composite was used to build the proximal wall against the metallic matrix band then cured and another single increment was placed to fill the rest of the cavity. The specimens were sectioned mesio-distaly using microtome followed by caries detecting dye application on each section. The dye was left for 15 seconds then the excess was removed with laboratory wipes.The sectioned specimens were examined using stereomicroscope and the images were analysed to detect the internal gaps. The length of the stained gaps was calculated as percentage (%) of the entire wall length.Data was statistically analysed using two-way ANOVA to determine the effect of tested restorative materials and application techniques on internal adaptation. Independent t-test was used to compare between the different restorative materials within each restorative technique. Significant level was set at p<0.05.The results revealed that both types of bulk fill resin composites (Admira fusion x-tra and X-tra fil) showed insignificant differences to each other when applied in bulk or split bulk technique. However, X-tra fil resin composite showed statistically significant higher % of gap formation when applied in centripetal technique compared to Admira fusion x-tra resin composite.