The present study clarifies feasibility of a new concept for the Flibe blanket system with Cesium from the thermonuclear viewpoint. Through this study it was revealed at first that the melting temperature of Flibe can be lowered with adding the CsF. And then the tritium breeding ratio when 137Cs was mixed in the Flibe was evaluated by using the neutron transport calculation code of MVP. The numerical results indicate that the TBR decreases gradually by adding Cs substituted for Beryllium.