INTRODUCTION: It is known that posterior composite restorations have high failure rates and high frequency of replacement as shown by studies. This may be attributed to the inability of conventional fillers to withstand the forces of mastication in the posterior region. New methods of reinforcement such as glass fibers are being used to increase the mechanical properties of dental composites. OBJECTIVES: was to compare the effect of aging in distilled water at 37°C for 1 day, 3 months and 6 months on the flexural strength and fracture toughness of a fiber reinforced composite (EverX posterior, GC, Europe), a nano-hybrid ceramic filled composite (IPS Empress Direct, Ivoclar Vivadent, Lieschtenstein) and a nano-hybrid zirconia filled composite (Z250 XT, 3M ESPE, USA). MATERIALS AND METHODS: For each test, twenty-one specimens were fabricated from each of the three composites and were then subdivided into three subgroups of seven specimens each according to the aging period in distilled water. After each aging period, the specimens were fractured in a Universal testing machine and the results were analyzed using ANOVA and post hoc test (Fisher's LSD) at p < 0.05 significance level. Following each test, the fractured surfaces of the 6 months aged specimens were examined using SEM. RESULTS: The flexural strength and fracture toughness of the fiber-reinforced composite was the highest with a statistical significance in the three aging periods followed by the nano-hybrid zirconia filled composite and the least was the nano-hybrid ceramic filled composite.SEM imaging findings were consistent with the results. CONCLUSIONS: The fiber-reinforced composite had the highest flexural strength and fracture toughness after each of the three aging periods Although aging in water decreased the mechanical properties of the fiber-reinforced composite, it still remained higher than the two nanohybrid composites which ensures its ability to withstand forces of mastication in the posterior region.