It is known that heat treatment decreases the hygroscopicity of wood. Thus heat-treated wood is good for outdoor construction due to its improved dimensional stability. This study is to investigate the hygroscopicity and discoloration of spruce and birch boards heat-treated above $190^{circ}C$. The equilibrium moisture contents (EMCs) at all relative humidities decreased with the increase of heat treatment temperature and/or time for both spruce and birch. It was revealed that heat-treatment temperature affected more on the hygroscopicity thaan heat-treatment time. The average basic densities decreased for the spruce specimens, but increased for the birch specimens with an increase of the treatment time and/or temperature. The same heat treatment condition (190-8) made the birch specimens darker than the spruce specimens.