Syudy on biodeversity of biodiversity of phytoplankton and macroalgae in Mae Sa Stream, Doi Suthep-Pui National Park, Chiang Mai province was carried out from March 1998 to September 1998. The samples were collected from five different sites along the stream. Fifty-nine species of phytoplankton were found and could be classified into 5 divisions : Cyanophyta, Crytophyta, Crytophyta, Crytophyta and Euglenophyta. The majority of the phytoplankkton were diatoms and the most abundant species were Fragilaria spp. And Navicula spp. Twenty-two species of macroalgae were found and could be classified into 3 divisions : Cyanophyta, Crytophyta and Rhodophyta. The most abundant species were Cladophora glomerata Hutz. Spirogyra spp., Rhizoclonium spp. and Oscillatoria spp. At the top of the stream, Bratrachospermun monifliforme Roth in division Rhodophyta was found, which was reported and collected once in Thailand by West and West in 1902.