Several models for dose survival curves in vitro had been proposed by many target, linear quadratic and so on. One of them is a linear quadratic (LQ) model which is described by the equation of E = e-(alpha d + beta d2). Also, the alpha/beta radio for radiation responses of normal or tumor tissues can be determined by their iso-effect relationships. Recently, the studies of LQ model in vitro and in vivo induced to new regimens of radiation therapy: hyperfractionation and superfractionation. The studies for cell growth in multifractionation indicated that the irradiated cells can grow within 4 fractions with daily dose of 2 Gy, thereafter an accumulation of G1 and G2 block were observed. However cell growing fraction seems to be relatively few in the irradiation conditions of more than 3 Gy of daily dose.