It has not been well explored how NPs may induce some adverse effects on the biological systems. In this research, the interaction of cobalt oxide NPs (Co₃O₄ NPs) with tau protein and PC-12 cell line, as nervous system models, was investigated with several approaches including fluorescence spectroscopy, CD spectroscopy, docking study, MTT, LDH, AO/EB dual staining, and flow cytometry assays. Fluorescence investigation displayed that Co₃O₄ NPs spontaneously mediate the formation of a static complex with tau protein through hydrogen bonds and van der Waals forces. Docking study also revealed that Ser and Gln residues play important roles in the formation of hydrogen bonds between tau and Co₃O₄ NPs. Far UV-CD measurement determined that Co₃O₄ NPs changed the unfolded structure of tau protein toward a more folded conformation. Moreover, Co₃O₄ NPs demonstrated to stimulate the reduction of PC-12 cell viability through membrane leakage, fragmentation of DNA, apoptosis, and necrosis. In conclusion, Co₃O₄ NPs may trigger marked alterations on the tertiary and secondary structure of tau protein. Also, the dose of Co₃O₄ NPs is the crucial factor which induces their adverse effects on the cells. Because, all side effects of NPs are not well explored, additional detailed experiments are more needed.