The Rapid drawdown condition is one of the most dangerous conditions for earth slopes. The change of water level in reservoir always causes a slope failure. This paper presents an investigation on the slope stability of the Rama 9 Reservoir under different drawdown conditions depending on the drawdown ratio, the drawdown rate and the loading conditions. Finite element analysis program PLAXIS 2D 2012 were applied to define the displacement and the factor of safety for slopes at selected positions. The results shown that, in the rapid drawdown condition for the drawdown ratio of 0.75, the factor of safety is 1.16. In the slow drawdown condition for the drawdown ratio of 1.00, the factor of safety is 1.26. Both are less than the ratio of allowable security value of 1.3. For the Rama 9 Reservoir, the reduction of the water level should be controlled more carefully.