Late Devonian (D3) silicate and Early Carboniferous (C1) silicate in Ailao Mountain belt, Three-River" area, are quite different from each other in mode of occurence, Radiolaria association, Si isotope and REE character. The former was formed in a deep_sea environment,i.e., in an intracontinental rift, whereas the latter is a component unit of Ailao Mountain ophiolite suite formed in a bathyal environment, indicating that Ailao Mountain was a small oceanic basin at that time.