Controllable etching of GaAs(100) has been electrochemically achieved on a nanometer scale by using a scanning tunneling microscope (STM) in acidic solutions (pH=2-3). The realized features on n-GaAs(100) surface were as small as 10 nm. We studied the dependence of the etching rate on the potentials applied to the STM tip as well as the GaAs substrate. These results indicate that the hole injection from the tip is responsible for the local etching of GaAs surfaces in electrolytes rather than local charges induced by an electric field.