Microemulsion (ME) is a one phase-transparent system, containing oil and water phases with surfactant/cosurfactant assistance to lower the surface tension. Recently, ME is widely used in biomedical application for prolonging drug delivery in human body. In this work, castor oil, tween 80 or tween 20 as the surfactant, glycerol or ethanol as the co-surfactant, and DI water as the water phase for ME preparation was studied by phase titration method. The effect of surfactant to cosurfactant ratio on the pseudo ternary phase diagram was investigated. A ratio of tween 80 to ethanol = 4/1 (w/w) resulted in the largest ME area in the pseudo ternary phase diagram. ME with tween as the surfactant and ethanol as the cosurfactant showed better stability according to their particle size and polydispersity index. It was found that 35-day storage and centrifugation did not have effect on the stability of the ME.
