This article investigates the changes in the Mongol- Yuan's modes of governing the Jinchi 金齒 and the Baiyi 百夷 in Yunnan from 1260 to 1381 utilising a recently discovered stele, the "Tomb Inscription for Instructor Mr. Kang" 教諭康公墓誌, dated 1461, other Chinese sources and Tai chronicles. It also examines how shifts in modes of governance affected existing power relations among tribal leaders, particularly the influence on the rise of the Baiyi ( a Tai group) and argue five points. First, the Mongol- Yuan gained possession of former Dali kingdom border areas in Yunnan by restoring political power to the deposed Duan royal family and appointed them as Native Officials to govern local society as the Duan Family General Administrator 段氏總管; Second, the Duan' s restoration aided the Mongol- Yuan advance into northern mainland Southeast Asia by providing them with access to communication routes leading from western Yunnan to the upper Ayeyarwaddy and Mekong river regions established by the Dali Kingdom; Third, at the beginning the Yunnan Branch Secretariat adopted direct military occupation as a means of administering border areas, so control of Jinchi and Baiyi tribal leaders aimed to serve military needs, resulting in their requisitioning the Duan Family General Administrator and submissive tribal leaders as guides and intermediaries for communicating with the Mian Kingdom ( Pagan) ; Fourth, though the military occupation accompanying the establishment of the Branch Secretariat for Conquering the Mian 征緬行中書省 at Tagaung in 1286/1287 ended up expelling the Mian from the upper Ayeyarwaddy, the mode of governance changed after the abolition of this Branch Secretariat in 1303. To control the southern border areas, the Mongol- Yuan now appointed Baiyi and Puman 蒲蠻 tribal leaders as Native Officials in 1327-1331, leading to Native Officials, including the Duan Family General Administrator, gradually becoming the main force for governance in border areas. The large Tai confederation of Mang2 Maaw2 ( Moeng Mao, Ch: Luchuan 麓川) suddenly appeared 1335-1350s by taking advantage of this change in mode of governance; Fifth, I cite the case of a Han Chinese man appointed as an official in the Pacification Office in Lan Na 八百等處宣慰司 1341 and other examples to demonstrate that the Yunnan Branch Secretariat and the Duan Family General Administrator supplied lower- level personnel to augment the administrative capability of Jinchi, Baiyi and Puman Native Officials.