Abstract: Rhamnolipid is a kind of biosurfactants.Experiments show that the microemulsion can be formed in rhamnolipid-n-butanol-n-heptane-water system.The quantity of NaOH and the critical micelle concentration(CMC) are determined when rhamnolipid was ionized entirely.The phase diagram of the system is obtained,and the formation and characteristic of phase microemulsion,interfacial tension and optimal salinity are affected by the concentration of NaOH in the system.Results illustrate that NaOH promotes ionization of carboxylic group,and increases electrolyte concentration.And when the surfactant is ionized entirely,the optimal salinity can reach the highest peak.