The phase behavior of the biodiesel/diesel-ethanol-water micro-emulsion system is investigated with chemical titration, and the effect of the biodiesel to diesel ratio on the phase behavior is analyzed.The results indicate that there exists an isotropic single-phase region in which the system formulates stable isotropic micro-emulsions consisting of biodiesel, diesel, ethanol and water without involving any surfactants, which reveals that biodiesel can be used successfully as a surface-active agent, and that the single-phase area depends on the concentrations of different components.At higher ethanol concentrations and higher ratios of biodiesel to diesel, the width of single-phase area is increased and the maximum limit of water dissolved in the mixture of biodiesel-diesel-ethanol which form a stable single-phase micro-emulsion is increased with higher biodiesel concentrations in the case of a constant ethanol quantity.Finally, the maximum limit of dissolved water in the mixture of different components is obtained.