Abstract: The simulation specimens of the fast breeder reactor' control rod were put into the thermal convection sodium loop, and the out of pile tests for B 4C/Na/S.S. system chemical compatibility were performed at 550 ℃, and the effects of the test period and oxygen content in sodium on the compatibility characters were investigated. The appearance of B 4C pellets is integral, and crack or break are not found after compatible test. There is B penetration in cladding inner surface, the amount of B penetration is propotional to the square root of the test period. The depth of B penetration is not changed with the add of the test period. The microhardness at the cladding inner surface increases clearly, its increment is increasing slightly with the add of the test period. The depth of the hardening layers is about 40 μm for different test periods. The reaction products Cr 2B, Na 4B 10 O 17, B 6Fe 23, CrB, Nib and NiB 12 are formed according to priority at the cladding inner surface for 80 to 400 d. The diffusion of B into the cladding relates to the temperature, test period and the oxygen content, and the oxygen promots the B diffusion.