Abstract: B 4C pellets with different B/C ratio and nuclear purity grade sodium were put into a stainless steel cladding tubes and the out of pile tests were carried out at 550,650 and 750 ℃. The test period is 82 d. After compatibility test, the appearance of B 4C pellets is integral, and crack or break are not found. However, the surfaces become rough and original metal luster is lost. The chemical reaction product NaB 5O 8 deposits and adheres to the surface of B 4C pellets. This results in the volume increasing of B 4C pellets. No considerable change in microstructure and grain size of B 4C pellets are observed. There are B、Na and C penetrate into cladding inner surface. The amount of B and Na penetration increases with the temperature and B/C ratio increasing, but C penetration is opposite. The chemical reaction products among Na, impurities in sodium, B 4C and cladding are NaBO 2, Cr 2B, Fe 2B and Ni 3B. The formation of the borides made increasing considerably in microhardness at the cladding inner surface.