This study evaluated the effects of boron impregnation and heat treatment on the chemical and mechanical properties of wood. Sugi (Cryptomeria japonica D. Don) sapwood specimens treated with either boric acid (BA) or di-sodium octoborate tetrahydrate (DOT) solutions were exposed to heat treatments at either 180 or 220 • C for 2 or 4 h. Chemical composition and strength properties were then measured in heat-treated specimens in comparison with untreated, unheated specimens. The wood carbohydrates were significantly degraded in the heat-treated specimens, suggesting depolymerization and alterations through the cleavage of acetic acid from the acetyl side chains. Increases in the amount of Klason lignin were found in heat-treated specimens, probably because of the ongoing removal of hemicelluloses during thermal degradation. A direct relationship was found between strength and hemicellulose losses of the specimens. As the hemicellulose content in the specimens decreased, losses in the modulus of rupture (MOR) increased. There were mixing effects of BA, DOT and thermal treatments on the chemical composition and strength properties, and it is therefore difficult to distinguish how these factors contribute to the properties evaluated. However, the effect of heat treatment on the mechanical properties was much clearer at the highest temperature (220 • C) than at 180 • C. The reductions in the modulus of elasticity (MOE) in the untreated wood were slightly higher than those in the BA-and DOT-treated specimens; however, this trend disappeared at treatments of 220 • C for 2 and 4 h, in which MOE losses reached 100% for all specimens. More detailed studies are needed to determine the effects of boron compounds on wood properties.