Abstract: Nanomaterials can modify the environmental behavior of arsenic (As) due to their large specific surface area and high reaction activity, which may affect the absorption and metabolism of As in microalgae. In this study, Chlamydomonas reinhardtii was used as model organism to investigate the influence of titanium dioxide nanoparticles (nano-TiO₂) on As(Ⅴ) accumulation and biotransformation in algal cells at different phosphate concentrations. The results showed that nano-TiO₂ significantly promoted As accumulation in algae cells in 0.013, 0.100 and 0.500 mmol/L phosphate groups at the beginning of exposure (1 d), but the carrier effect of nano-TiO₂ decreased with the extension of exposure time. After 8 days of exposure, in the nano-TiO₂ addition groups, As(Ⅴ) in algal cells was not only reduced to As(Ⅲ) and methylated to dimethyl arsenic, but also further transformed to an unknown As compound, possibly arsenosugars. And the proportion of arsenosugars in algal cells gradually increased with the decrease of phosphate concentration, which might inhibit the efflux of As(Ⅲ). After 8 days of exposure, As(Ⅴ) and As(Ⅲ) were the main As species in the culture medium, and a small amount of dimethyl arsenic was also detected. The addition of nano-TiO₂ decreased the proportions of As(Ⅲ) in the culture medium, especially in 0.5 and 1.0 mmol/L phosphate group. This study indicated that the interaction between nanomaterials and phosphate significantly affected the accumulation and metabolism of As in microalgae, which facilitates the application of microalgae in As remediation.