The purpose of this study was to investigate the effect different physical energies, either heat or sonic activation, on the surface microhardness of two different bulk-fill resin composites. Materials and Methods: Sixty disc shaped resin composite specimens with standardized diameter of 6mm and overall thickness of 4 mm were used in this study. The prepared specimens were divided into two equal groups (N=30), according to type of bulk fill resin composite (R); Group 1: X-tra fil (Voco) (R1) and Group 2: SonicFill 3 (Kerr) (R2). Each group was further subdivided into three equal subgroups according to the method of energy (E) delivered to the bulk fill resin composite material before light initiated polymerization (n=10); either without any energy delivered to the material (Eo), preheating at 54℃ using resin composite heating device (Calset, AdDent Inc, Danbury, CT, USA) (E1) or sonic activation (SonicFill™ hand piece, Kerr, USA) (E2). Specimens were cured from the top using light curing unit for 40 seconds. Finally, the VHN (S) was evaluated at the top (S1) and the bottom (S2) surfaces of each specimen. Results: Three-way ANOVA revealed statistical significant differences in two main effects; the types of resin composites (P<0.0001), and tested depth of restoration (P<0.0001). On the other hand, methods of physical activation showed no statistical significant differences (P = 0.631). Conclusions: Bulk fill resin composite formulation is a major determinant of its Vicker's hardness number. Both sonication and pre-heating had the same effect on VHN of nano-hybrid, SonicFill 3 bulk fill resin composite.