BaTiO3 thin films on MgO encapsulated GaAs(100) were prepared epitaxially, in condition ranging from 600A°C to 800A°C and from 2 x 10-4 Torr O2 to 1 x 10~2 Torr O2, by pulsed laser deposition. The epitaxial relationship between all materials was cube-on-cube. BaTiO3 thin films obtained were c axis oriented tetragonal phase and showed ferroelectric properties. Epitaxial BaTiI 3 had quite smooth surface of less than 100 A in roughness. BaTiO3 grown on 40-A-thick epitaxial MgO was also epitaxial although defect density in BaTiO3 looked higher than that in BaTi03 grown on 400-A-thick MgO. The interfaces of BaTi03/40-A-thick MgO/GaAs structure were abrupt and free of secondary phases in spite of the quite thin MgO layer. © 1993 The Japan Society of Applied Physics.