Adjacent tropical south and southeast Yunnan have a similar monsoon climate and are covered by tropical rain forest vegetation at lowlands. Both of the floras of tropical south and southeast Yunnan have tropical floristic elements comprising a majority (78.3% in south Yunnan and 68.83% in tropical southeast Yunnan), and those of tropical Asian distribution make  up the highest ratio (30.2% of the total genera in south Yunnan and 27.3% in southeast Yunnan), and both of them belong to Indo-Malaysian flora at northern margin of tropical Asia in present floristic regionalization. However, some temperate East Asian characteristic families, such as Magnoliaceae, Cornaceae, Smilacaceae, Theaceae, Styracaceae, Symplocaceae, Aquifoliaceae, Caprifoliaceae and Celastraceae, etc. are well represented in the flora of tropical southeast Yunnan while tropical Asian characteristic families, such as Meliaceae, Annonaceae, Menispermaceae, Zingiberaceae, Apocynaceae, Asclepiadaceae, etc. are well represented in the flora of south Yunnan. Although the tropical south and southeast Yunnan share 80% of their genera, 237 genera are seen only in south Yunnan, and 349 genera only in tropical southeast Yunnan respectively. Furthermore, 57 genera of East Asia distribution, 53 genera of North temperate distribution and 22 genera of endemic to China, as well as 17 genera of East Asia and North America disjonct distribution are present only in tropical southeast Yunnan. These suggest that the flora of southeast Yunnan could have different background from the flora of south Yunnan in origin. The flora of tropical southeast Yunnan is more related to Eastern Asian flora in origin, while the flora of south Yunnan is more related to Indo-Malaysia flora.  It is therefore suggested that there could be a biogeographical line between south and southeast Yunnan. The possible biogeographical line is here named as “Hua line”. The biogeographical line is well supported by geological history of the region.