Abstract: In this work, the dynamic moisture adsorption processes and influence factor analysis of moisture adsorption property of graphite (IG-110) and boron carbide (BC) were studied, which are significant for moisture proof and dehumidification in high temperature gas-cooled reactor (HTR). The micropore structures were studied by mercury porosimetry, N₂ adsorption and SEM, and the surface functional groups were characterized by XPS. BC possesses a large amount of closed pores, with abundant opening pores above 5 μm and slit at mesoporous level; whereas most pores in IG-110 are opened with the size between 1.5 μm. According to this study, the size and the amount of pores are both key factors to moisture adsorption properties. The equilibrium moisture adsorption content of BC is obviously larger than that of IG-110, with a rapid growth of moisture absorption content for both materials in the initial 2 h. Under different moisture conditions, the absorption equilibriums of two materials are generally achieved in 5-8 h, and the equilibrium moisture absorption contents are linearly with the humidity in the range of 60%-90% at 25 ℃.