Abstract: The gas phase thermal charging method was performed for SA508-Ⅲ steel with H charging at high temperature and high pressure. The interaction between H and the tensile deformation behavior was studied at room temperature to clarify the hydrogen-induced brittleness mechanism and provide the basic data for the steel design of nuclear power plant. The results showed that the yield strength of the steel with H charged slightly increases， and the reduction area obviously decreases. The fracture morphology changes from normal micro-void coalescence fracture to mixture of river pattern and dimple fracture due to H charging. The increase of the yield strength ascribes to the pinning effect of H on dislocation，thus the initiation of dislocation can be resisted.During the plastic deformation process， H moves following the moving dislocations and concentrates at the interfaces between the carbides and matrix. Once H concentration reaches the critical value， the bonding force between the carbides and matrix decreases， resulting that the ductility of the steel decreases.

Key words: SA508-Ⅲ steel, hydrogen embrittlement, carbides, tensile deformation, dislocation