To evaluate fertilization of the Mekong River flood and inundation, we observed nutrient concentrations of flood water and nutrient amounts of soil at paddy fields located in the Mekong River floodplain and calculated nutrient balance. We obtained the following conclusions; 1) Sluice gate is more effective system than rubber gate at the point of nutrients utilization because water at the level of a lower layer have much nutrients due to the deposition. 2) Nutrient balance calculation expresses that the amounts of nutrients contained in soil and inundation water are sufficient to provide for rice production. 3) The contribution ratio of flood to rice production that is presented in "the nutrients amount brought by inundation / the nutrients amount included in the harvested rice" are 98%, 50%, 88% and 17% about nitrate at Kandal, phosphorus at Kandal, nitrate at Takeo and phosphorus at Takeo respectively.