[en] B₄C 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 degree C. The test period is 82 d. After compatibility test, the appearance of B₄C 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 NaB₅O₈ deposits and adheres to the surface of B₄C pellets. This results in the volume increasing of B₄C pellets. No considerable change in microstructure and grain size of B₄C 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, B₄C and cladding are NaBO₂ , Cr₂B, Fe₂B and Ni₃B. The formation of the borides made increasing considerably in microhardness at the cladding inner surface