We have succeeded in growing epitaxial (0001) MnSb/(111)B GaAs films with an atomically flat heterointerface using a GaAs buffer layer. For MnSb films with a thickness of 2–20 Å, the in-plane strain in the films arising from a lattice mismatch between MnSb and GaAs is compressive, which was observed byin situreflection high-energy electron diffraction measurement. On the other hand, a rough interdiffused interface was obtained in samples grown without a GaAs buffer layer. In contrast to the case of samples with flat interfaces, the in-plane strain is tensile, which is attributed to interdiffusion of Sb into GaAs substrates. The magnetization of the MnSb films with the thickness of this range shows a strong dependence on the interface quality. The saturation magnetization and the saturation magnetic field of the samples with the rough interface are smaller than those of the samples with the atomically flat interface. ©1997 American Institute of Physics.