Abstract: The chemical compositions and structures of different kinds of bio-oils were compared by gas chromatography/mass spectrometry (GC/MS) and Nuclear Magnetic Resonance Spectroscopy (NMR). It was shown that contents of main kinds of compounds were significantly different in different kinds of bio-oils because of the discrepancies of chemical compositions of biomasses. Bio-oils from corncob and pine contained more aromatics, and that from straw had more anhydrosugars, aldehydes and ketones, while that from bagasse contained more organic acids, aldehydes and ketones. ¹³C-NMR spectra revealed that bio-oil from straw contained the most alkyl carbon and carbonyl carbons, while that from pine and bagasse contained the most aromatic carbon and carboxylic carbon, respectively. From the ¹H-NMR spectra, it was observed that the most alkyl hydrogen and sugar hydroxyl hydrogen were found in bio-oil from bagasse and straw, respectively. While the most phenolic hydroxyl hydrogen and aromatic hydrogen both existed in the bio-oil from pine. The discrepancies of functional groups of different kinds of bio-oils resulted in the differences of physical properties.