The Chao Phraya River basin (CPRB) in Thailand had a massive flooding in 2011, which affected on not only Thai socio-economics but also global supply-chain of industrial production. The Thai government is planning watershed management for the CPRB. In order to propose a new reservoir operation system on science-based which can be easily put into practice, we studied a reservoir optimum operation for Bhumibol and Sirikit reservoirs in the CPRB. This paper proposed the new reservoir operation method that daily release discharge were decided by using the day before storage volume data. A usefulness of the reservoir operation method was verified using historical hydrological data. As a result, the proposed reservoir operation method could contribute stabilization of storage volume.