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Heat treatment is one effective method of improving dimensional stability and durability of wood. However, it usually decreases the bending strength of wood. Vacuum heat treatment, which is one way that does not use a thermal medium, had little influence on wood elasticity or bending strength in our previous study. But until now, few studies have reported on the changing mechanism of vacuum heat–treated wood. In this article, chemical composition and crystallinity of untreated and treated Eucalypt pellita wood were investigated under vacuum heat treatment temperatures from 80°C to 280°C for 4 hours. The results showed that lignin content increased and that holocellulose and α-cellulose decreased when the temperature was raised. Alcohol benzene extractive increased at the initial stage and then decreased when the temperature was above 240°C. Crystallinity of wood increased with a temperature increase from 80°C to 200°C and reached a maximum value of 52.57 percent. Fourier transform infrared spectroscopy analysis indicated that absorption intensity of a benzene skeleton at 1,600 cm−1 increased and that the hydrogen bond absorption band at 3,423 cm−1 shifted to a lower wave number as temperature increased.