The electronic structure of M’ type GdTaO4 is studied by first-principle pseudopotential calculation within the frame of density-functional theory. The calculated band structure of M’-GdTaO4 revealed that the top of the valence band is dominated by O-2p and the bottom of the conduction band is dominated by e orbits of Ta-5d. The spin-up and spin-down electrons of Gd-4f are located at 627 eV below the top of the valence band and at 301 eV above the bottom of the conduction band when on-site Coulomb interaction Ueff=8 eV is applied. The calculated refraction index of M’-GdTaO4 is 224 which is in good agreement with the result abtained from the Gladstone-Dale relation.