An electrothermodynamic analysis of field evaporation in the configuration of scanning tunneling microscope(STM) has been performed. By introduction of an effective binding energy, a formula for the evaporation field in STM has been constructed. The calculated evaporation field for Au-1, for the first time to our knowledge, agrees well with experiment. On the other hand it was found that the classic charge exchange model and image hump model yield very high evaporation field because they require that the adatoms be fully ionized and evaporated to infinity.