Nanometer-sized Au (gold) dots on a Si substrate were formed by field evaporation using an atomic force microscope (AFM) in air. Theoretical and experimental studies show that field evaporation is possible in the AFM metal probe/vacuum/thin insulator/conductor configuration. Theoretically, the field evaporation is generated by further application of an additional bias voltage less than 30-40 V in a thin insulator. Experiments demonstrated that ultrasmall gold dots were formed on a natural SiO2/Si substrate by the applied,voltage less than 10 V, and ultrasmall gold dots of 15 nm in diameter were made on the insulator.