[en] The survival fraction in HeLa cells irradiated by low dose X-rays was observed using clone method. The results showed that the survival fraction in the cells irradiated by less than 0.5 Gy X-rays was higher than control, 'hormesis' of HeLa cell survival was obtained and was significant at doses near 0.25 Gy; also, the damage degree of cells induced by the following irradiation was reduced because of pre-treating the cells with low dose D₁ of 0.05, 0.75 Gy; it was found from above that 'adaptive response' of cell survival was induced by the low dose irradiation