Titanium dioxide nanoparticles (TiO2-NPs) have been found to translocate into the brain by penetrating the blood-brain barrier (BBB), but it remains largely unknown how their physicochemical characteristics may impact BBB permeabilization. By testing TiO2 particles of different shapes and various sizes, we found that: (1) small, spherical TiO2-NPs permeabilized a BBB-like human brain microvasculature endothelial cell monolayer better than rod-like or large particles; (2) TiO2-NPs stimulated F-actin stress fiber formation, and induced paracellular gaps and ROCK II activation. The TiO2-NP-mediated BBB permeabilization was associated with intracellular uptake and cytoskeletal re-organization; and (3) in rats, spherical, small TiO2-NPs significantly increased the BBB permeability and entered the brain. The TiO2-NPs were accumulated in the brain, but no obvious pathological anomaly was observed in the cerebral cortex and hippocampus. Our study investigated the neurotoxicity of TiO2-NPs, thereby providing scientific evaluation for the potential biomedical applications of TiO2-NPs.