Flood benefit was calculated for integrated evaluation of the Mekong River flood and inundation in Cambodia. Flood and inundation were simulated using dynamic wave model in main channels and non-uniform flow model for flooded area. Agricultural, industrial and fishery benefits were mainly considered. Data were obtained from field and literature survey. The estimated benefits were about 20 billion-FY for rice production and 10 billion-FY for fish catch in the case of the 2000 flood. On the other hand, the damage to rice production was only about 5 billion-FY. Supposed that upstream countries developed flood control system and could make 50% water level reduction, damage decrease would be less than all benefits decrease. This means that flood and inundation bring much benefit in the Lower Mekong. We would like to propose the development with allowance of flood and damage.