This work has been devoted to water valuation regarding Vietnamese reservoirs in a competitive electricity market environment. Water value concept, which has been assumed as fuel cost of water, helped to reflect the exploiting cost of water resource in the context of hydrothermal power system. In this work, the Stochastic Dual Dynamic Programming (SDDP) approach has been suggested for modeling the water valuation problem. Further, a trade-off has been obtained between im mediate and future operating cost. Effectiveness of the developed approach has been investigated in Vietnamese electric power generation market for the period of 2011-2012. The results have been obtained in terms of the water value for major reservoirs, short-run marginal cost (SRMC) and classification of generating units as well.Moreover, optimal generation scheduling of cascading reservoirs and their respective trajectories have also been analyzed in this work.