Abstract: O/W microemulsions was prepared together with water, as well as evening primrose oil as oil phase, with tween 80 and lauric acid as surfactant. Dynamic light scattering and transmission electron microscope (TEM) was used to measure the size and shape of microemulsion droplets. The entrapment eficiency were determined to check the stability of the microemulsions by ultraviolet spectrophotometry. Through particle size, encapsulation efficiency and storage test, the influence of pH, temperature and time on the storage stability of microemulsions were examined. The results showed the microemulsions were stable at pH5 ~ 7. The microemulsions were shown to be stable after stored at 4℃ and 20℃ for 30d. No delamination and demulsification was observed, and the entrapment eficiency was above 92.21%, 91.89%. The microemulsions were unstable storing at 60℃ and the entrapment concentration declined within 30d.