Studieds of the modification mechanism of Au and Ti surfaces by electric fields applied with a scanning sunneling microscope (STM) are carried out at atmospheric pressure and room temperatute. For pulses of 1-100μs, nanodots can only be formed on Ti under conditions where the current shows substantial fluctuation, indicating tip-sample contace. If the field is applied gradually, Au is not affected and pits are formed on Ti. We interpert pit formation as a result of fiele-induced oxidation. Moreover, experiments show that the relative humidity plays an important part in nanostructure formation on Ti. Our results can be explained through a combination of direct contact and oxidation inducedby electricf field.