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In order to understand the Neogene vegetation succession and climatic changes in Southwest China, we reconstructed the Middle Miocene vegetation and climate at four localities, which are at different latitudes (24°19′25.3″N / 23°35′10.2″N / 22°27′44.5″N / 21°17′05.9″N) along the SW side of the Ailao Mountains in Yunnan Province.The palynological assemblages obtained from the four localities suggest that the Middle Miocene vegetation there was composed of mixed evergreen and deciduous broad-leaved forests with some coniferous forests growing under subtropical conditions.Based on the palynological data, seven paleoclimatic parameters of the four localities are obtained by applying the method of Co-existence Approach and compared with the modern and the Late Miocene and Late Pliocene climatic parameters in Yunnan.The comparison revealed that: 1) Among the four palynological assemblages, the abundance of angiosperms increase while those of gymnosperms and pteridophytes decrease along the gradient of latitudes from north to south.2) The mean annual temperature (MAT) and mean coldest monthly temperature (MCMT) were obviously lower in Middle Miocene than today (MAT: 13.2 to 14.6 comp.17.7 to 21 ℃ ; MCMT: 2.9 comp.10.9 to 15.2℃).It suggests that the Ailao Mountains were not high enough to block the summer monsoon and the winter monsoon.3) In contrast to the Neogene global cooling, the regional values of the MAT and MCMT in Yunnan increased and accompanied by a decrease of the difference of temperature between the coldest and warmest months (18.4℃ to 19.1℃ comp.9.5℃ to 12.3℃) since Middle Miocene.This may be linked to a gradual reduction in the strength of the Asian winter monsoon as the Tibetan Plateau was uplifted.