Objectives: The goal of the present study was to assess the effect of titanium dioxide (TiO₂) nanoparticle (NPs) on the fracture toughness, surface microhardness of an experimental resin based composite (RBC). Methods: A light curing resin matrix was made by mixing 50wt% Bis-GMA and 50wt% TEGDMA.  Silane coated SiO₂ filler (mean particle size: 0.5µm, 5-15nm) was added in the ratio of 65 wt% of the resin composites.  Acrylic acid modified TiO₂ NPs (21 nm) were prepared and added with the ratios of 0, 0.2, 0.3 and 0.4wt% to the experimental resin composites. Fracture toughness was assessed utilizing a universal testing machine. Surface microhardness was assessed utilizing Vickers microhardness tester. Results were analyzed using ANOVA test. Results: RBCs-containing 0.2, 0.3 and 0.4wt% TiO₂ NPs had superior fracture toughness and surface microhardness values in comparison to TiO₂-free RBC. However, this improvement decreased gradually for RBCs-containing 0.3 and 0.4wt% TiO₂ NPs.  Significance: RBC-containing 0.2wt% TiO₂ NPs is a hopeful material with enhanced mechanical properties that can be used for higher stress-bearing restorations such as Class I and II. Keywords (experimental composite, titanium dioxide nanoparticles, experimental resin based composite).