Abstract Cosao is a Tibeto-Burman language of the Lolo-Burmese branch spoken in three villages that straddle the border China-Laos border. It is spoken by about 150 people in the village of Man'gang in Mengban Town, Mengla County, Yunnan Province, China, as well as by approximately 400 speakers in two villages across the border in Phongsaly Province, Laos. Lexical data of Cosao (autonym: tsho55 sɔ55) has only recently been published in Bai Bibo (2015). Bai (2015) recognizes that Cosao is a Lolo-Burmese language, but provides no arguments about where exactly Cosao fits within the Lolo-Burmese branch. Using comparative lexical evidence, this talk explains how Cosao is a Southern Loloish language most closely related to the Khir language of Laos, and also shows how Sida, Sila, Phusang, Khir, and Cosao all constitute a cluster of closely related languages within the Southern Loloish branch. Computational phylogenetics is also used to show the position of this cluster of languages within Southern Loloish.