The structural and optical properties of Ga2Se3 on (100)GaP and (100)GaAs prepared by molecular beam epitaxy have been investigated. The electron diffraction studies revealed that the superstructure was formed in [011] direction by the spontaneous ordering of native gallium-vacancies in the defect zinc blende structure under a selenium-rich growth condition, and very large absorption anisotropy (Δα>104cm1) was observed in the vacancy-ordered Ga2Se3. Furthermore, polarization dependence of photoconductivity due to the absorption anisotropy was observed in the vacancy-ordered Ga2Se3 on (100)GaAs.