We have investigated the mechanism for the abnormal oxide growth occuring during oxidation of the crystalline tungsten silicide. TEM and XPS analysis reveal the abnormaly grown oxide layer consisting of crystalline $Wo_3$ and amorphous $SiO_2$. The presence of crystalline $Wo_3$ provides a rapid diffusion of oxygen through the oxide layer. The abnormal oxide growth is mainly due to the poor quality of initial oxide layer growth on tungsten silicide. Two species such as tungsten and silicon from decomposition fo tungsten silicide as well as silicon supplied from the underlying polysilicon are the main contributors sto abnormal oxide forma-tion. Consequently, the abnormal oxidation results in the disintegration of tungsten silicide and thinning of polysilicon as well.