We have calculated the optical properties(the imaginary part of the dielectric f unction ε2(ω),the optical absorption coefficient I(ω),the reflectiv ity R(ω)) of tetragonal and cubic crystals of KTa0.5Nb0.5O3 by the full potential linearized augmented plane wave method (FLAPW).Our calcul ated tetragonal optical spectra show three big peaks around 3,7 and 23 eV along a and b separately.Along c,there are three mainly peaks at 4,8,23 eV respectively,among which the one at 4 eV is sharp and high.Comparing optical pr operties in the tetragonal and cubic phass,we concluded that the tetragonal phas e has more anisotropic optical responses.