The aim of the present study was to determine the effect of sonic vibration on the adaptation to cavity walls and margins and the depth of cure (assessed through microhardness testing) of Sonic-Fill and Bulk-Fill resin composite restorations after aging for 24 hours and 12 months.The materials used were SonicFill (Kerr) resin composite and X-tra fil (Voco) resin composite. A total number of 80 standardized Class II cavities were prepared on both mesial and distal wall of total number of 40 premolars. The teeth were randomly divided into two equal groups, of 20 teeth each, according to the material; either SonicFill resin or X-tra Fil resin composite. Each group was divided into two equal subgroups, of 10 teeth each, according to sonic vibration applied; either application of resin composite using sonic vibration or without sonic vibration. Each subgroup was divided into 2 classes, of five teeth each (with 10 proximal restorations), according to the aging duration; either after 24 hours or 12 months in artificial saliva at 37◦C. In all specimens Single Bond universal adhesive (3M ESPE) was applied following manufacturer’s directions in selective-etch mode, followed by placement of SonicFill and X-tra Fil materials in bulk (5mm). The teeth were then sectioned into 2 halves, each half used for assessment of adaptation or depth of cure through microhardness. Each cut tooth half was then sequentially finished and polished, and ultrasonically cleaned. For adaptation assessment, the pulp chambers of each cut half were blocked, a small amount of caries detecting dye was traced over the restoration and the internal margins. The specimens were observed under Digital Microscope at 70X connected to computer with image analyzer software. The adaptation results were presented as gap percentage per tooth wall by calculating the axial and gingival gap length divided by the total length of the wall multiplied by 100. Surface area of voids, if present, were also calculated per specimen. The Depth of cure was assessed indirectly using Vicker’s microhardness test with load of 300 gm for 10 seconds. HV hardness scale was used at two depths from the occlusal surface (1 mm and 4 mm depth). Two VHN readings were recorded for each depth and the average was calculated. Depth of cure percentage was calculated by dividing bottom depth micohardness in relation to top depth multiplied by 100 for each specimen separately. Results were statistically analyzed with three-way ANOVA and post-hoc tests (p>0.05). Concerning adaptation, the results of the present study revealed that, X-tra Fil composite material showed more gap formation than SonicFill, with the use of sonic vibration at the axial wall of the cavity after 24 hours aging duration. The same results was revealed at the gingival wall, even without sonic vibration, at 24 hours. While for vibration and aging, there was no significant effect on adaptation to axial and gingival walls. Regarding voids, the results showed that SonicFill revealed less voids than X-tra Fil when applied without vibration. With sonic vibration, the results revealed reduced total voids in the X-tra fil restoration, compared to without vibration. Aging had no effect on resultant findings regarding void formation. While for depth of cure, the results of the present study showed that the percentage of Bottom/Top microhardness of the two investigated materials in all situations was above 80%. In addition, there was also no statistical significant difference between all the groups regarding depth of cure. Sonic vibration is recommended with viscous bulk-fill materials to decrease internal void formation. Also, Bulk-fill materials could be safely used in deep cavity preparations without compromising the depth of cure. Durable restorations could be expected, in view of the maintained adaptation and depth of cure, as well as the improved microhardness of these materials by time