Abstract: OBJECTIVE To investigate the phase behavior of microemulsion, frequently-used emulsifier and co-emulsifier's solubilization ability with oleic acid as the oil phase and find the best quaternary system for water solubilization and oleic acid solubilization. METHODS Pseudo-ternary phase diagrams were constructed to screen the emulsifier, co-emulsifier and mass ratio for both by water solubilization parameters (A_T, W_m, S_m) and oleic acid solubilization parameter (O_m). O/W microemulsion area in phase diagram was determined by conductivity and phase inversion point.RESULTS The pseudo-ternary system was apt to form gels and the amount of water solubilization was limited with polyalcohol as co-emulsifier. Tween 20 had the maximum amount of water solubilization, but tween 80 had the best O_m; the amount of oil/water solubilization was partly related to the HLB value. When using tween 80/absolute alcohol as mix-emulsifier, O_m was in direct to their mass ratio, but water solubilization was not completely; the biggest A_T was in 1∶1 mass ratio, but the O/W microemulsion area was limited. CONCLUSION The quarternary system of olelic acid, water and tween 80/ethanol with a mass ratio equal to 1∶1, has a clear O/W structure and proper amount in water and oleic acid solubilization.