This study of the diversity of phytoplankton and benthic algae in Mae Sa stream, Doi Suthep-Pui National Park was carried out from April 1997 to February 1998. Eighty seven species of phytoplankton were found which could be classified into 5 divisions, 8 oders, 19 families and 31 genera. The majority of the phytoplankton were diatoms in the Order Pennales and the most abundant species were Melosira varians Agardh, Fragilaria ulna (Nitzsch) Lange-Bertalot, Cymbella tumida (Brebisson) Van Heurck and Nitzschia linearis (Agarth) W. Smith. There were 172 species of benthic algae found. The most abundant were also diatoms in the Order Pennales. The majority of the species belonged to the genera Navicula (38 species), Nizschia (23 species), Fragilaria (16 species) and Gomphonema (15 species). The most abundant species were Navicula lanceolata (Agardh) Kutzing, Nitzschia dissipata (Kutzing) Grunow, Nitzchia palea (Kutzing) W. Smith Fragilaria capucina Desmazieres, Gyrosigma scalproides (Rabh.) Cleve Coconeis placentula Ehrenberg, Alchnanthes lanceolata (Brebisson) Grunow, Cymbella tumida (Brebisson) Van Heurck, Gomphonema augur Ehrenberg, Gomphonema parvulum (Kutzing) Grunow, Surirella capronii Brebisson and Surirella capronii Kutzing.