Abstract: Heat treatment of Pinus sylvestris and Quercus mongolica boards were carried out in atmospheric stream or 0.45 MPa pressurized steam. Hygroscopicity and dimensional stability of the treated wood were compared with the control. The results showed that heat treatments not only lowered the equilibrium moisture contents(EMC)of the wood, but also altered the pattern of its moisture adsorption in high humidity environment. When the environmental relative humidity increased from 69% to 94%, the EMC change rates of the heat treated samples were just one third of those of the control samples. This might be attributed to the reconstruction of wood cell wall during the heat treatments. Dimensional stability of heat treated samples was significantly improved as a result. The radial and tangential swelling of pressurized-steam treated Pinus sylvestris samples decreased by 34% and 47% compared to those of the control ones. For Quercus mongolica samples, the corresponding values were 46% and 51%. Most tangential anti-swelling efficiency(ASE)values of the heat treated samples were higher than their radial ASE values. That means swelling difference between radial direction and tangential direction was also reduced after heat treatments. It was also showed that pressurized steam induces more pronounced property modification than atmospheric steam.