The relationship between the volume (V) and the water level (Z) is a significantfactor in calculating the reservoir balance for reservoir operation management. Obtaining this factor often requires numerous measurements which is costly and such data must constantly update due to natural and artificial impacts (i.e. landslides, sedimentation, sand mining,...). In addition, errors in measurement and data processing have the great effect on the accuracy o f data. Therefore, the adjustment o f V - Z relationship o f a reservoir is extremely necessary for accurately forecasting hydrological process and reservoir operation management. In this study, 06 optimal methods, including: Square method, Rosen Brock method, Nelder - Mead method, Hook - Jeeves method, GA method, and Shuffled Complex Evolution - SCE method were tested to evaluate their advantages and disadvantages and then applied the V -Z relationship adjustment for Ban Chat, Huoi Quang, Hoa Binh and Thac Ba reservoirs.