At present, ecosystem function in Lake Ulak Lia can be found as i.e.: recreation area, fisheries, and retention basin of Musi River. Meanwhile, the basis data of these ecosystem function was limited and difficult to assess so far. Therefore, initiate study compose of plankton composition, tropic level, and water quality of Lake Ulak Lia was compulsory. Based on the index diversity of the plankton; it was indicated that Lake Ulak Lia was have had lightly to intermediate disturbance, in addition related to the tropic level, the lake can be classified as oligarchical (nutrients poor) to tropic level. Furthermore, the water quality of this lake was appropriate for recreation area and fisheries except parameters pH, lead, and copper.