The aim of this study was to investigate the improvement in bio-oil stability via hydrodeoxygenation (HDO) with noble metal catalysts. The bio-oil produced by fast pyrolysis was subjected to the hydrodeoxygenative upgrading process with Pd/C, Ru/C, or Pt/C at 300 degrees C for 60 min with 3 MPa H-2 pressure. Upgraded bio-oil (heavy oil) was stored in sealed glass bottles at 23 degrees C for 2, 4, 8, or 12 weeks. The major properties (water content, viscosity, and acidity) of heavy oils after storage and the chemical features (elemental composition and functional groups) of phenol polymers in the aged heavy oil were characterized. Most properties of the aged heavy oil were maintained at the initial levels during the storage period, which indicates that the oil properties were stabilized by HDO. The water content slightly increased from 0.3-2.1 wt% to 2.3-3.5 wt% during the storage, but the viscosity and acidity of the oil hardly changed. Chemical compositions of low-molecular weight compounds in the aged heavy oils were slightly modified during storage. A slight increase in molecular weight was also observed and was attributed to partial repolymerization of phenol polymers in the heavy oil during storage. Overall, this study suggests that HDO upgrading effectively improves the stability of bio-oil properties. (C) 2016 Elsevier Ltd. All rights reserved.