Heat treatment is becoming increasingly popular and is growing as an industrial process to improve wood properties. The Finnish wood heat treatment technology, ThermoWood, is the most commonly used technology in the industrial area. In this study, pyrolysis of untreated and heat-treated ash wood (Fraxinus excelsior L.) was carried out using a fixed-bed reactor at different pyrolysis temperatures. The influences of the heat treatment process and the pyrolysis temperature on the yields and chemical composition of products were investigated. The maximum bio-oil yield was 46 wt% at a pyrolysis temperature of 550 degrees C for untreated wood, while the biooil yield was found to be about 41 wt% at a pyrolysis temperature of 500 degrees C for heat-treated wood. The elemental composition and higher heating value (HHV) of the bio-oil was determined. The chemical composition of the bio-oil was investigated using some chromatographic and spectroscopic methods, such as gas chromatography-mass spectrometry (GC/MS) and 1 H-NMR. It was found that the heat treatment process significantly reduced the organic acids, ketones and aldehydes, while it increased the phenolic compounds. These results show that heat-treated wood sawdust should be used as a valuable feedstock for production of the bio-oil.