In this study the crystallinity and estimations of relative triclinic (I-alpha) and monoclinic (I-beta) structure content of cellulose isolated from heated spruce (Picea orientalis) and beech (Fagus orientalis) wood samples were determined by using Fourier Transform Infrared (FT-IR) spectrometry. Heat treatment was applied on the test samples in an oven at three different temperatures (150, 180 and 200 degrees C) and two different durations (6 and 10 h) under atmospheric pressure. It was determined that crystallinity of cellulose in wood samples increased with thermal modification. The results indicate that the changes in crystallinity of cellulose in wood samples related to not only temperature but also time during thermal modification. I-alpha/I-beta ratio of cellulose in spruce and beech wood samples changed with thermal modification, but it was established that monoclinic structure was dominant in cellulose crystalline structure. It was designated that the crystalline structure of cellulose in spruce wood samples affected from thermal modification more than in beech wood samples. (c) 2005 Elsevier Ltd. All rights reserved.