The scanning tunneling microscope has been used to modify the surface of YBa2Cu3O7-x (YBCO) high T-c. superconducting thin films by operating the instrument in the so-called mechanical milling or field-induced evaporation mode. Nanostructures such as holes, lines, and trenches were fabricated on the YBCO thin film surface in a controlled manner. In the surface modification and nanostructure fabrication processes, the effect of bias voltage, tunneling current, and scanning feedback control parameters on the modification efficiency have been investigated.