Abstract: A large application of nano-materials increases their risk entering the marine environment.The primary purpose of this study is to explore the toxic effects and mechanisms of nano-TiO₂, which is widely applied in industry and commercial products.Algal growth inhibition test was carried out, and chlorophyll concentration and lipid peroxidation level were measured using Skeletonema costatum and Dunaliella salina as model organisms.Moreover, adsorption of nano-TiO₂ on microalgae was observed by SEM technique.The settlement of nano-TiO₂ particles in experimental medium with 24 h was also characterized.Results showed that both growth and photosynthesis of S.costatum and D.salina were inhibited by nano-TiO₂.Toxic effects of nano-TiO₂ on S.costatum were similar to those of D.salina.Adsorption of nano-TiO₂ and oxidative damages resulting from interactions between nano-TiO₂ and microalgae could inhibit growth and photosynthesis of microalgae, which were the main reasons for toxic effects of nano-TiO₂.The settlement of nano-TiO₂ particles in experiment medium caused the decrease of suspension concentration which had an influence on results.