Abstract: The comparative observation by electron microscopy between three cases of giant pigmented hairy nevus and four cases of common pigmented nevus, all were intradermal nevus, was reported. The characteristics of ultrastructure in the former disease showed large melanosomes including compound melanosomes, more compound melanosomes in special shape, and the appearance of medium electron-dense spherical bodies in places where melanosomes aggregated. These data suggested that there was higher activity of nevus cells in giant pigmented hairy nevus which strengthened the ability of producing more melanosomes, perhaps increased melanoprotein. These data also enlightened us why this disease was liable to malignancy.