Resin composite preheating is an innovative method that could be clinically beneficial by improving the handling properties, marginal adaptation, and surface properties of uncured nanofilled resin composite materials. There is conflict and unclear information regarding the effect of preheating on the microhardness, fracture toughness and surface roughness of nanofilled resin composites. Thus, it is important to assess whether dental clinicians can adopt preheating procedures without compromising composite mechanical strength. Objective: The purpose of this study was to evaluate the effect of preheating on microhardness, fracture toughness and surface roughness of nanofilled resin composite. In this study, one commercial nanofilled resin composite Filtek Z350 XT was used. A total of 28 disc-shaped specimens were fabricated in a Teflon mold (10 mm diameter x 2 mm thick) for Vickers microhardness indentation test and surface roughness test. The samples were divided into two groups of 14 samples each, one group of samples was light-cured at room temperature (24ºC) without preheating (non-heated group), and the other group was light-cured after preheating (preheated group). Vickers hardness measurements of 14 specimens (n=7) either preheated or non-heated of the top and bottom surfaces was measured by means of microhardness tester by applying 100 g load for 10 s. Surface Roughness measurements (Ra) were obtained from 14 specimens (n=7) either preheated or non-heated with the atomic force microscope. Fourteen single-edge-notched-beam specimens were prepared for fracture toughness test (n=7) either preheated or non-heated with measurements (2.5 x 5 x 25 mm3) and a crack 2.12 mm in length. The specimens were tested via three-point bending mode, using a universal testing machine at crosshead speed of 1.0 mm/min until failure occurred. Independent sample t- tests revealed no significant difference between non-heated and preheated groups for all tests (p>0.05). However, for Vickers hardness test, there were significant differences between top and bottom surfaces for non-heated and preheated groups (p<0.05). Moreover, surface roughness average Ra (nm) mean values of preheated group was higher than non-heated group but no significant difference between them was found (p>0.05). Preheating procedure did not negatively affect microhardness, fracture toughness and surface roughness of nanofilled resin composites so preheating is recommended for the other potential clinical advantages.