Single-crystal films of high-T_C superconductors are of immense importance for fundamental physical studies and the device applications. The in-situ growth of the single-crystal films of YBa₂Cu₃O_7-x (YBCO) at relatively low temperatures is reviewed. The (001)-oriented single-crystal films of good quality in crystallinity have been prepared by activated reactive evaporation and they exhibit the superior superconducting properties. The orientation of the film grown on SrTiO₃ (110) face was discussed in relation to the lattice mismatch between the substrate and the film. It is also discussed that the crystal transformation of YBCO with oxidation had the effect on the microstructure and the oxidation degree of the film. The in-situ reflection high energy electron diffraction observation during depositon on MgO (100) suggests that the deposition occurs in the layer-by-layer growth manner. Some of the superconducting and physical properties measured using our films are reported.