objectives: Nanoindentation technique was used to evaluate the effect of thermal aging on Young's modulus (E) and nanohardness (H) of bulk‑fill and fiber reinforced resin composites. Materials and methods: Three resin composites that fall into the categories of conventional (GrandioSo, Filtek Z350 XT; Z250), bulk‑fill (tetric Evo ceram, flowable SDR Flow) and fiber reinforced (Ever-X posterior, Alert) were evaluated in this study. Ten disc specimens (8 mm × 2 mm) were prepared from each material. The E and H were determined by a nano-mechanical tester (UMT 1, Bruker, Santa Barbara, CA, USA) equipped with a Berkovich diamond indenter tip both before and after thermocycling 5,000 times in distilled water (5°C-55°C). Data were collected and Statistically analyzed using one‑way analysis of variance and paired t‑test. Results: All materials were significantly different from each other regarding Hardness before and after themalcycling with the highest means for GrandioSo and Alert. It was found that GrandioSo and Alert had the highest modulus values. But no significant differences between Tertic evo ceram, Z250, and Ever X. All of the evaluated composites showed a significant drop in hardness as a result of thermal cycling, while only a minor reduction(insignificant) in elastic modulus was observed. Conclusion: E and H were significantly increased when the filler content increased. H was significantly affected by thermal cycling. E was slightly affected by thermal cycling.