This study aims to investigate the Effects of Titanium dioxide nanoparticles (TiO NPs) on the stereological 2 parameters in the dentate gyrus and the morphology of granular hippocampal neurons in adult mice. Adult male mice (n=20, weight average: 45 g) were randomly divided into four groups including: group receiving saline (controls), low-dose (LD) 2.5 mg/kg TiO TiO 2 NPs, medium-dose (MD) 5 mg/kg TiO NPs and high-dose (HD) 10 mg/kg TiO NPs, daily using gavage for 35 days. To estimate the 22 volume of the hippocampus, dentate gyrus, and sub-layers of dentate gyrus the Cavalieri principle was used. The physical dissector was used to determine the numerical density of dentate gyrus granular cells. For analyzing the morphology of dentate gyrus granular cells the qualitative Golgi staining was used. Our data showed that the total volume of the hippocampus, dentate gyrus and its sublayers including molecular, granular and polymorph in TiO treated mice decreased significantly compared to the control group. Moreover, the total 2 number and numerical density of dentate gyrus granular sub layer cells showed a significant reduction in all three experimental groups compared to the control group. The granular cells of the dentate gyrus had shorter dendritic length and decreased dendritic branches in the TiO -treated in comparison with the control mice. These data can justify the disorders related to memory, learning and hippocampus 2 neurons damages due to using of TiO2NPs.