By gradually varying the bias voltage between the gold tip and the gold sample, it is observed with the scanning tunneling microscope that large bias voltages may induce atoms to diffuse on the gold tip and ofter result in an increasing of the tip length. We also observed the field emission and resonant tunneling phenomena. A mechanism based on the polarization of the tip is proposed for the field-induced atomic diffusion. Such field induced diffusion is believed to be responsible for surface modification with large bias voltage pulses.