Objective Applying the in vivo Mammalian Alkaline Comet Test， this study aimed to investigate the DNA damage caused by the food additive titanium dioxide and nano-titanium dioxide on rat liver cells.Methods According to the guideline “In vivo mammalian alkaline comet assay” （OECD TG489）， male SD rats were randomly divided into nine groups，five rats per group. Food additive titanium dioxide was administered to three groups at doses of 1000， 500 and 250 mg/kg·BW， while pure water served as the negative control. Titanium dioxide nanoparticle was administered in three different doses： 500， 150， and 50 mg/kg·BW. The negative control group received 0.8% Tween + 3% FBS. Test substances were administered by gavage once a day for 15 d. Simultaneously， a positive control group was established， and the positive substance ethyl methyl sulfonate （EMS） was administered once daily on the 14^th and 15^th days via gavage， at a dosage of 200 mg/kg·BW·d. Animals were anesthetized six hours after the last administration，and the livers were harvested to prepare single-cell suspension smears. The sections were subsequently subjected to lysis，unwinding， electrophoresis， staining to acquire the comet images for analyzing.Results There were no statistical differences （P>0. 05） in the percentages of tail DNA content between the treated and negative control rats. However， the percentage of tail DNA content of rat liver cells in the EMS group showed a significant difference compared to the negative control group （P<0.01）.Conclusion Under the circumstances of our test， neither the food additive titanium dioxide nor nano-titanium dioxide caused the DNA damage in rat liver cells.