Abstract: The objectives are to explore the toxic effect and mechanism of nanomaterials, and provide a basis for environmental toxicity monitoring and evaluation of nano titanium dioxide (TiO2) and nano zinc oxide (ZnO). Protozoa Paramecium caudatum was took as an experimental object, and the acute toxicity experiment method was used to test the 24 h half effect concentration (EC50) and the 24 h lowest effect concentration (LOEC) of TiO2 and ZnO. Under EC50 concentration stress of Paramecium caudatum for 24 h, the activity changes of the three kinds of antioxidant enzymes were compared, the morphology of the two kinds of nanomaterials was observed by SEM. The biotoxicity mechanisms of the two nanomaterials were analyzed. The results showed that the 24 h-EC50 was 51.580 mg/L(TiO2) and 0.444 mg/L (ZnO), respectively, and the lowest effect concentration (24 h-LOEC) was 1.712 mg/L(TiO2) and 0.352 mg/L(ZnO), respectively. The changes of antioxidant enzyme activities were as follows: superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities changed to different degrees compared with the control group. The difference significance analysis indicated that the two kinds of nanomaterials showed a more obvious inhibition on POD enzymes of Paramecium. These results suggest that the 24 h-acute toxicity of nano-ZnO on Paramecium is greater than that of nano-TiO2, and the toxicity of nanomaterials to Paramecium caudatum is related to its inhibitory effect on antioxidant enzymes.