[en] Studies on carcinogenic effect of tritiated water is introduced in two parts. The first part is an in vitro study in which CHL-1 cells were exposed to tritiated water (9.25 x 10⁵ ∼ 3.5 x 10⁶ Bq/ml) for 24 ∼ 96 h and the accumulated dose was from 0.055 to 0.88 Gy. In order to estimate RBE of tritium for malignant transformation in CHL-1 cells, the induction of malignant transformation in CHL-1 cells by exposure to gamma rays of ¹³⁷Cs was tested. Based on the transformation rates, the RBE of tritium for malignant transformation in CHL-1 cells was estimated to be 1.6. The second part is an in vivo study. In the study, rats were fed with tritiated water (2.22 x 10⁵ and 1.11 x 10⁵ Bq/ml) for 1.5 a. Rats in control group were fed with tap water. Results showed that in the statistics, the differences in the total tumor incidence and malignant tumor incidence between high and low dose rate groups and control groups were remarkably significant