The optical constants were calculated for coherently strained alloys GexSi1-x on Si (001) substrates with tight - binding method. The effects of strains on the electronic energy band structures are taken into account by modifying the tight-binding parameters according to the variations of the bond angle cosines and the bond lengths. For the changes of the bond lengths, an empirical scaling rule is used, in which the scaling indexes are determined through fitting the calculated values of deformation potential constants to their experimental values for Ge and Si. The momentum matrix elements in the formula for calculating the imaginary part of dielectric constants are determined by fitting the ε2 of Ge and Si. The calculated results for the optical constants , including the imaginary part of dielectric constant, the refractive index, the absorptance and the reflectance, are given both for the strained alloys and bulk alloys GexSi1-x as x=0.2 and 1. By comparison of them, the effects of strains on the optical constants are discussed.