Malignant transformation of golden hamster embryo cells was induced by thorium dioxide consisted of ,228,Th and ,232,Th in equal quantity. The final concentration of thorium dioxide was 3 or 6 µg/ml medium, special activities of radiation were 0.01065 or 0.0212 Bq and accumulative average absorbed dose for each cell was 4.24×10,-12,—3.38×10,-9, mGy.,Morphological transformation foci appeared 15 days after thorium dioxide added. Cell mutation was observed in ouabain-resistant assay. Maximum saturation density of transformed cells was increase 4.4 times as comparing with control group. The transformed cells acquired the agglutinability following exposure to concanavalin A. Increase of the microvilli on the surface with various thickness and formation of blebs and ruffles were observed. The transformed cells gained the ability of forming colonies in soft-agar medium 33 days after exposure to thorium dioxide.,The above results indicated that malignant transformation of golden hamster embryo cells was directly induced by thorium dioxide in vitro.