We have demonstrated that an etched feature as small as 10X10 nm was realized on n-GaAs surfaces in a Ni-salt solution by using a scanning tunnelling microscope (STM). Injected carriers from the tip were responsible for the local anodic etching. in the same solution, we deposited a micron-size Ni island on n-GaAs. An electric held between the tip and the substrate induced the local Ni deposition.