The toxicity of arsenic (As) can be influenced by many environmental factors. Among them, nanomaterials can adsorb arsenic and alter its bioavailability in organisms. However, the studies on long-term effects of arsenic in the presence of nanoparticles are limited. Thus, the 21-d effect of titanium dioxide nanoparticles (nano-TiO) on chronic toxicity of arsenic (arsenate and arsenite) was investigated in two generations of Daphnia magna. The exposed concentration of nano-TiO was 1 mg/L and the concentration of As(Ⅲ) or As(Ⅴ) was 0.2 mg/L which was lower than the 48 h-NOEC (no observed effect concentration). The survival, body length, average number of offspring and time of first brood were determined. Our results indicated that the exposure to nano-TiO and As during the parental generation can affect the health of offspring. Nano-TiO was found to significantly alleviate the mortality and reproduction inhibition of As on D. magna, and the alleviation of As(Ⅴ) was more prominent than that of As(Ⅲ). It is likely that nano-TiO alters the metabolism and adsorption condition of arsenic in the gastrointestinal tract of D. magna. Overall, these results indicate that the increase of arsenic adsorption onto nano-TiO in the gut of D. magna could alleviate the toxicity of arsenic. Nonetheless, further research should be conducted to study the influence of arsenic on the multi-generations of aquatic organisms, especially when it is coexisted with other substances.