Ga2O3(Gd2O3)-GaAs heterostructures in situ fabricated using a multichamber ultrahigh vacuum (molecular beam epitaxy) system were studied by x-ray reflectivity measurement and high-resolution transmission electron microscopy. The oxide-GaAs interfaces were found to be very smooth with the roughness no more than 1 nm. Moreover, an interfacial roughness as small as one atomic layer of GaAs (0.33 nm) was observed using x-ray reflectivity.