Superconducting YBa2Cu307_ x (YBCO) thin films on MgO (100) substrates were successfully fabricated by a reactive evaporation method under oxygen plasma conditions. In this paper, details of the reactive evaporation method and the optimum preparation conditions are described. The superconducting transition temperature, surface morphology and crystal quality were examined as a function of the substrate temperature Tsu b and the oxygen partial pressure Po2. Oxygen was introduced into the film through an injection nozzle. After deposition, the films were allowed to cool with the same pressure as used in the deposition under reactive oxygen plasma. The as-grown films showed Tc .... = 83 K with Tco,~_e ~= 88 K under the preparation conditions of Tsu b = 620°C and Po2 = 1.4 x10 torr. X-ray diffraction analysis showed preferentially c-axis orientated superconducting phases perpendicular to the film surface. The crystal quality and degree of preferred orientation of the films were improved as the substrate temperature increased. The as-grown films were post-annealed at various temperatures and examined for changes in Tc .... and lattice parameter c; the values of Tc .... were not affected by the post-annealing, although the lattice parameter c was reduced by high temperature annealing above 750°C. These results may be related to the deficiency of oxygen in the CuO2 plane of the YBCO perovskite structure.