Microemulsion technique is employed to reduce the viscosity of vegetable oils, which is used as the source of renewable fuels. A recent study is conducted by Ren zhu et al., on the phase behavior of microemulsion and the solubilization mechanism of water and castor oil, in order to evaluate the solubilization capacity of water and castor oil in diesel under given conditions. The proper concentration of rhamnolipid (RL) was 50 g/L. N-octanol was certified as an ideal cosurfactant with the optimal cosurfactant/surfactant (C/S) mass ratio (w/w) of 0.60. The optimum castor oil/diesel (V/D) volume ratio (v/v) was 0.18. Moreover, fuel properties of water-containing castor oil/diesel (WCD) microemulsion were identified. The thermal and storage stability of WCD microemulsion were also conducted. Compared with castor oil, WCD microemulsion has lower viscosity, which presents similar fuel characteristics as diesel.