This study of Green Algae, Family Desmidiaceae that can be indicators of clean water, was undertaken from 9 sites in the Upper Part of Northern Thailand between August to October 1998. They could be found in all of the reservoirs that were studied. The dominant species found in Chiang Saen lake was manfeldtii var. fluninense Schumacher, Mae Ngud Somboonchai Dam was Cosmarium moniliforme (Trup.) and Mae Sa stream was Cosmarium moniliforme (Bory) Ehrenberg ExRalfs. These species can be used to indicate oligotrophic or clean water. Other reserviors such as Ang Kaew reserviors, Huay Tung Tao reservoir, Huay Mae Yen reservior, Nong Bua reservior, Mae Kuang Udomtara Dam and Kwan Pha Yau reservior that are all mesotrophic desmids can be found example for Euastrum sp., Staurastrum spp., Staurodesmus spp. And Pleurotacnium sp. In smaller amounts.