Abstract: AIM：To study the formation of microemulsion phases at 25℃ for systems containing soybean phosphatidylcholine and the effect of formulation variables on the physical-chemistry characterization of microemulsion. METHODS: High purity lecithin (PC>96%) was used as surfactant, several short chain alcohols (i.e. ethanol, n-propanol, n-butanol, n-pentanol) were investigated as cosurfactant and isopropyl myristate as oil. Viscosity, conductance, refraction and mean droplet diameters of the microemulsions were measured. RESULTS: In each system studied, w/o microemulsion could be observed along the surfactant /oil axis. All microemulsions were stable at room temperature for several months. The physicochemical properties of lecithin microemulsions were influenced by the formulation variables: conductance and mean droplet diameter increased as the proportion of water increased; the proportion of water and the ratio of lecithin/ethanol (K_m) showed predominant influence on the viscosity of the microemulsion. CONCLUSION: The extent of the regions of the microemulsion was dependent upon the nature of the cosurfactant and the oil and also the lecithin/cosurfactant mixing ratio (K_m). The IPM/PC/ethanol/water system was chosen to produce O/W microemulsions used for drug delivery. The formulation that has relatively large K_m and medium proportion of water was better to be used in drug delivery system.