The oxidation properties of Si nanostructures on SOI substrate were quantitatively evaluated by novel microscopic methods. Si structures embedded in thermal oxide were non-destructively observed through the oxide layer using scanning electron microscopy and atomic force microscopy. A pattern-dependent oxidation (PADOX) of the Si nanostructures on SOI substrate was quantitatively evaluated by use of these techniques. PADOX is combined phenomena, suppression of oxidation by mechanical stress and oxidation from under Si layer caused by oxygen diffusion in the buried oxide layer. As a result of co-occurrence of these two phenomena, the local oxidation rate strongly depends on the shape and size of the Si nanostructures.