Economic growth of the Greater Mekong Subregion has been drastically increasing in the past few years. Especially as the role of the Mekong River and its tributaries become more important, therefore the evaluation of hydrological data will be necessary for infrastructure development in the future. The time-series variation of long term hydrological data such as daily precipitation and discharge which are available in Mekong River Commission was evaluated by using the method of regression analysis, excess threshold analysis and probability analysis. As a result, the annual precipitation and the average of discharge have been on a declining trend in southern region of the Lao PDR especially rainy season. On the other hand, the average of discharge in the tributary which has hydro power dam such as NamNgum, Sekong, Sedone has been increasing trend in the dry season. It is also understood that fluctuation range of flood and drought discharge is increasing compare with last 40 years.
