Ab initio theoretical computations have been performed to reveal the mechanism of the etching reaction of silicon oxide (SiO2) by HF molecules. The probable reaction paths, in which the etching reaction proceeds through four sequential steps to remove a single fragment of SiO2, are presented with their potential energy curves. In every step, the insertion of an HF molecule into an Si-O bond leads to the dissociation of the Si-O connection. The potential energy barriers evaluated along the reaction paths suggest that the HF molecule has an ability to etch the SiO2 surface. The strong interaction among HF molecules, however, likely causes HF polymer formation, which is expected to reduce the reaction rate of SiO2 etching.