Objectives: The purpose of this study was to evaluate the effect of the introduction of nanofillers on the mechanical properties of fiber-reinforced composites (FRCs) for stabilization and conservative treatment of multiple traumatized anterior teeth. In particular, the aim of the research was to point out the force levels of two sizes (diameters 0.6 and 0.9 mm) of both conventional and nanofilled FRCs. Methods: Eighty FRCs samples were divided into eight groups, each consisting of 10 specimens. Conventional (groups 1, 2, 3, and 4) and nanofilled (groups 5, 6, 7, and 8) FRC samples were evaluated. Each FRC was tested in two diameters (0.6 and 0.9 mm) and under two deflections (1 and 2 mm). Each sample was polymerized with the same halogen curing unit and then evaluated with a 3-point bending test on a universal testing machine after 48 h of dry storage. Results: Nanofilled FRCs showed significantly higher load values than conventional FRCs. Moreover, 0.9-mm-diameter FRCs showed significantly higher load value than 0.6-mm-diameter FRCs. Specimens tested at 2-mm deflection showed significantly higher load values than those tested at 1-mm deflection. Conclusions: Nanofilled FRCs showed significantly higher load values than conventional FRCs. Higher flexural strength values were recorded with 1-mm deflection for both FRC tested.