In this paper, the authors report on the investigation on the effect of inserting an epitaxial MgO layer on the structural and magnetic properties of Fe₃O₄ ultrathin films grown on GaAs(100) by combined techniques of molecular beam epitaxy and postgrowth annealing in an oxygen atmosphere. The MgO films were grown with a cube-on-cube orientation to GaAs. The epitaxial Fe₃O₄ lattice cell was rotated by 45^deg in respect to that of MgO and GaAs due to the lattice relaxation of the magnetic layer. All samples studied exhibited strong fourfold cubic magnetic anisotropy with easy axes along the lang001rang directions of GaAs. A dramatic decrease in the uniaxial anisotropy constant in the presence of the MgO barrier was observed. The existence of the uniaxial magnetic anisotropy in the Fe₃O₄/GaAs hybrid structure has been attributed to the interfacial contact between the magnetite film and the GaAs.