Newly identified radiolarians from ribbon chert in the Mae Hong Son-Mae Sariang area, northwestern Thailand covered Early Carboniferous, Late Permian, and Middle-Late Triassic in age, which indicate that there was a pelagic basin during the Late Paleozoic and Triassic in this region to- gether with the published radiolarian biostratigraphic data. This basin is joined with the Chiang Dao and Changning-Menglian oceanic basins, which represent the main oceanic basin of Paleotethyan Archi- pelago Ocean. The main oceanic basin was situated in the traditional “Shan-Thai Block”. Therefore, “the Shan-Thai Block” was not a single block during that stage, but composed of the Paleotethyan Ocean and two continental terranes that affiliated to Gondwana and Cathysian domains respectively. Newly identified radiolarians from ribbon chert in the Mae Hong Son-Mae Sariang area, northwestern Thailand covered Early Carboniferous, Late Permian, and Middle-Late Triassic in age, which indicates that there was a pelagic basin during the Late Paleozoic and Triassic in this region to gether with the published radiolarian biostratigraphic data. This basin is joined with the Chiang Dao and Changning-Menglian oceanic basins, which represent the main oceanic basin of Paleotethyan Archi- pelago Ocean. The main oceanic basin was situated in the traditional “Shan -Thai Block ”. Thus, “ the Shan-Thai Block ”was not a single block during that stage, but composed of the Paleotethyan Ocean and two continental territories that affiliated to Gondwana and Cathysian domains respectively.