Epitaxial growth of (001) oriented YBa₂Cu₃O_7-x(YBCO) on SrTiO₃(100) and MgO(100) by activated reactive evaporation was investigated by insitu reflection high energy electron diffraction (RHEED). RHEED observation of the YBCO films on both the substrates demonstrated the formation of the perovskite structure even at initial deposit. A sharp streak in every deposition stage revealed the layer-by-layer growth. The initial deposit on the substrate of MgO with the large mismatch of 9% kept the same in-plane lattice spacing as MgO. The lattice spacing converted from the bulk value of Mg0 to that of YBCO when the layer became thicker than 12Å.