Abstract: The food grade olive oil microemulsion system was prepared by using soybean lecithin and span80 as mixed-surfactants,ethanol as a cosurfactant. To optimize the formula and explore the formation rule of the olive oil microemulsion,the effects of many factors on the phase behavior of the olive oil microemulsion were investigated by pseudo ternary phase diagram,such as hydrophilic lipophilic balance(HLB)values,the quality ratios(Km)of the mixed-surfactants and cosurfactant,the addition of chloride sodium and so on. The structural transition of the microemulsion system was determined by conductivity and negative-stained transmission electron microscopy. The results showed that larger single-phase microemulsion region was obtained by mixing soybean lecithin and Span80 as surfactants at a constant weight ratio of 3:1 and keeping the quality ratio of mixed-surfactants and ethanol at 2:1.The addition of glycerol or a small amount of inorganic salt in the aqueous phase could expand the single-phase region of microemulsion system to some extent,while excessive salt ions(C_NaCl>4 mol/L)would decrease the microemulsion region. In soybean lecithin/Span80/olive oil/ethanol/glycerol/water microemulsion system,glycerol was mixed with water as the aqueous phase at a constant weight ratio of 1:1.As the aqueous phase weight content increased beyond 22 wt%,the system gradually transformed into bicontinuous structure from W/O type. The phase separation appeared once the aqueous phase dilution exceeded 30 wt%. Therefore,this study provided theoretical and technical supports for preparation and application of the olive oil microemulsion.