Recently, the widespread use of TiO 2 nanoparticles (nano-TiO 2) has attracted the global concern regarding their adverse effects to aquatic organisms. Nanoparticles may get to the aquatic environments and harm the growth of algae. This paper presents the studies of nano-TiO 2 concentrations influence on the photosynthetic characteristics, antioxidant system and antitumor activity of marine macroalgae Gracilaria lemaneiformis after exposed to 0, 5, 10, 20 or 40 mg L −1 of nano-TiO 2. Results showed that the photosynthetic pigment content of G. lemaneiformis obviously decreased and the antioxidant defense system was significantly disturbed after exposure to nano-TiO 2. Compared with control, the maximum level of malondialdehyde showed 3.3-fold increase for 15 days exposure to nano-TiO 2. The activity of superoxide dismutase and catalase enhanced obviously by 1.10–2.24-fold. The aqueous extract of G. lemaneiformis (AEGL) possesses anticancer activity against HepG2 liver cancer cells proliferation and the 48 h-cell inhibition exceeds 35%. But the toxicity of nano-TiO 2 would inhibit the anticancer activity of AEGL. When G. lemaneiformis was exposed to nano-TiO 2, the effect of the aqueous extract of G. lemaneiformis on the cell inhibition, reactive oxygen species production, apoptosis, cycle arrest of HepG2 cells significantly lessened, and which were linearly associated with nano-TiO 2 exposure concentration. Those results further confirmed that nano-TiO 2 can inhibit the anticancer activity of G. lemaneiformis. The results in this work imply that macroalgae can be an effective biomarker of nano-TiO 2 contaminant, and can be useful indicator to evaluate ecological effects of increasing pollutants in marine ecosystem.