<p>This work studied the effect of the silica and functionalized silica nanoparticles on the density, hardness, and water absorption of the polymer epoxy nanocomposites. Silica nanoparticles were synthesized by the sol–gel method and further functionalized with epichlorohydrin (EPR) and furfuryl amine (FA) by post-synthesis grafting technique. Different samples of nanocomposites with filler loading of 0.5 wt.% of silica, EPR-functionalized silica, and FA-functionalized silica in epoxy nanocomposites were prepared. FESEM image analysis was done to understand the surface morphology and to confirm the functionalization of silica nanoparticles. The Shore D hardness test was performed to check the hardness of the nanocomposites. Density, hardness, and water absorption rate of these samples were tested and compared with neat epoxy (NE). Tensile test and TGA analysis were performed to check the mechanical and thermal behavior of the nanocomposites. The density of the epoxy gat increased by ~ 3% by adding 0.5 wt.% filler. Epoxy with functionalized silica as the filler has more density. By adding silica and FA-functionalized silica, the epoxy’s hardness gets improved by ~ 2% and ~ 4%, respectively. The rate of water absorption had been reduced by ~ 30% by adding fillers to the epoxy resin system. There is an improvement in tensile strength of ~ 57%, ~ 35%, and ~ 67% due to the reinforcement of 0.5 wt.% of silica, epoxy functionalized silica and furfuryl amine functionalized silica, respectively with respect to neat epoxy. TGA analysis shows improvement in thermal stability due to the addition of functionalized nanofillers. These changes are related to the better interfacial adhesion and the covalent reaction of the nanofiller with the epoxy matrix system.</p>