Wear behavior between a Si tip and a SiO2 film in KOH solution at various pH values has been examined by using an atomic force microscope. We found that the Si tip removal amount strongly depended on the solution pH value and was at a maximum at pH 10.2-12.5. This result indicates that wear behavior of the Si tip is similar to that of actual chemical mechanical polishings of a Si wafer. It was also found that the Si removal volume in moles was approximately equal to that of SiO2 irrespective of the solution pH value. This equality implies that a Si-O-Si bride is formed between one Si atom and one SiO2 molecule at the wear interface, followed by the oxidation of the Si tip, and finally;he bond rupture by the tip movement and the silica species including the Si-O-Si bridge is dissolved in the KOH solution.