Abstract: The Fraxinus mandshurica board was treated with superhigh temperature in atmospheric pressure superheated steam kiln and pressure steam tank respectively. The effects of two different treatments on the equilibrium moisture content (EMC), dimension stability, sorption hysteresis and shrinkage coefficient of the treated board were investigated. The reason for improving dimension stability by two treatment methods was further revealed. The results show that both of the two treatments can reduce the EMC of the wood, but the pressure steam treatment is more efficient than another one. That method has an advantage to reduce the change of wood moisture content due to the ambient relative humidity, reducing the shrinkage coefficient and increasing the dimension stability. An increase in the heattreat temperature leads to a decrease of the sorption hysteresis of the heattreated wood. The sorption hysteresis of atmospheric pressure superheated steam kiln is lower than that of pressure steam tank under the same heattreat temperature.