Bio-oil is liquid fuel produced by fast pyrolysis, typically, of biomass. Bio-oil comprises a mixture of highly oxygenated compounds, carboxylic acids and trace water. Upgraded bio-oil can be used as a substitute for conventional fuels. However, bio-oil is inherently unstable. The various compounds in bio-oil can react through many chemical reactions, such as polymerizations, during the storage of bio-oil, resulting in adverse changes in the bio-oil's properties, especially increasing viscosity over time. In the present study, three sets of methods to improve the bio-oil's stability were investigated: addition of antioxidants, addition of solvents, and emulsification with conventional hydrocarbon fuels. In the first set of methods, three kinds of antioxidants (propyl gallate, tert-butyl hydroquinone, and butylatedhydroxyanisole) were added in 1000-ppm concentration to bio-oil. In the second set, 10wt of solvents, including acetone, biodiesel, ethanol, ethyl acetate, and methanol, were added to the bio-oil. Finally, the third set involved emulsification of bio-oil with different conventional hydrocarbon fuels, including diesel, gasoline, and biodiesel, using octanol as a surfactant. All test samples were subjected to accelerated aging, involving exposure to high temperature of 80°C for 5 days. The viscosity of the samples, chosen as the main indicator of the aging, was measured daily. The results showed that under the accelerated testing conditions, pure bio-oil aged significantly, with 44.65 increase in viscosity. The bio-oil with antioxidants, on the other hand, aged more slowly, with 17-20 viscosity increase. The addition of solvents also slowed down the aging drastically, especially in the case of biodiesel, with only 4.91 viscosity increase. Emulsification with conventional hydrocarbon fuels also showed promising results, with similar trends to those of antioxidant and solvent addition. All results showed that the three sets of stabilizing methods can improve the bio-oil's stability significantly, with slightly varying degree of effectiveness. Selection of an optimal method in practice depends on the particular constraints and circumstances of each operation. © 2011 IEEE.