This research presents a study on slope stability analysis at Nong Loeng Pueai Detentions Basin, Khalasin Province Project. The main objective is to evaluate the factor of safety of the slope in two conditions, including at the normal water storage level, and rapid drawdown. The researcher investigated slope failure in the filed by measuring the dimension, height, and depth of the slope failure. Soil profiles at four point of slope failure were investigted by kunzelstab and field investigation. The soil samples were collected and soil properties for each testing points was tested in laboratory. The slope stability analysis were conducted the GeoStudio2007 (Slope/w) with Ordinary Method of slice, Simplified Bishop, Simplified Janbu and Morganstren-Price method. After investigation, the result showed that the soil at area could be classified into four zones including, silty clay, sand, clay and sand. The factor of safety at the normal water level minimum ranged between 1.199 and 1.454 while the factor of safety at rapid drawdown minimum ranged between 0.769 and 0.861. The slope stability analysis implied that slope at Nong Loeng Plui is instability and need to the strength of subsoil needed to be improved.