The pseudomorphic octahedral chlorite and serpentine in chromitite of western Ali, Tibet were previously discussed by the authors (1983). Recently, they have found pseudomorphic octahedral diopside, tremolite, and chlorite in serpentinized harzburgite of the same area. The Ca-rich and Al-poor characteristics of octahedral diopside suggest a metasomatic reaction under the condition of lowering temperature and pressure. This is consistent with the characters of cell parameters of octahedral diopside. The Si-rich and Al-poor features of octahedral chlorite are similar to conditions of octahedral diopside, but are completely different from the chemical characters of hypabyssal hydrothermal chlorite. The octahedral diopside is also obviously distinct from diopside of hypabyssal altered ultramafite in chemical composition. The chemical fratures of these two octahedral silicates may be attributed to hydrothermal metasomatism during the early stage uplifting and emplacement of the mantle rock. The reactions of ringwoodite-octahedral diopside-octahedral tremolite and ringwoodite-octahedral serpentine-octahedral chlorite are dealt with in this paper.