Among forest diseases, fungi are the most important agents that cause irreparable losses to the wood of standing trees and logs. In this study, pyrolysis of undecayed and decayed beech (Fagus orientalis) wood were carried out using a fixed-bed reactor at pyrolysis temperature of 500 degrees C in nitrogen atmosphere. The influence of Trametes versicolor fungal decay on the yield and chemical composition of products was investigated. The bio-oil yield was 62.5 wt% at a pyrolysis temperature of 500 degrees C for decayed wood, while the bio-oil yield was found to be about 58 wt% at the same temperature for undecayed wood. Bio-oils were characterized using some chromatographic and spectroscopic techniques, such as gas chromatography-mass spectrometry (GC/MS). It was found that Tetracosamethyl-cyclododecasiloxane (5.50%), tetradecamethyl-hexasiloxane (4.85%), 2,6-dimethoxy-phenol (4.21%), and benzene acetic acid (3.16%) were the main oil components present in decayed beech wood, while syringol (14.86%), methoxyeugenol (6.59%), naphthalene (4.41%), o-guaiacol (3.60%), isoeugenol (3.17%), and 2-methoxy-4-methyl-phenol (3.08%) were present in undecayed beech wood. These results show that decayed wood can be used for the production of bio-oil.