Abstract: H terminated n Si(111) surface was found to be etched in 0.5% HF solution in nanometer scale while scanning the STM tip when the substrate potential was somehow lower than the open circuit potential at tunneling condition of V b=+1.0 V (tip positive), I t=1 nA. Much more stable Si surface was achievable at potentials much more negative than the open circuit potential (such as 500 mV negative to OCP) under the same tunneling condition. Furthermore, the etching rete at less negative potential was found to be somehow related to the tunneling current. These results indicate that the STM induced local modification of Si(111) surface is electrochemically controllable. A proposed model of the hole injection through the tunneling current is put forward to account for the induced etching of Si surface.