Objective: The purpose of this study was to determine the effect of increasing concentrations of the combination of tween 80 and ethanol to the formation and stability of the physical properties of eucalyptus oil microemulsion. Methods: This oil is formulated into a microemulsion by increasing the concentration of the combination of tween 80 and ethanol (1: 1) with various concentrations, ie 40%, 45%, and 50% as a component of surfactant and cosurfactant. The evaluation was done for 8 weeks covering organoleptic test, test phase sepa ration, viscosity test, measurement of pH, density measurements, measure ments of surface tension and particle size measurement. Results: The pH of micro-emulsion showed in the range 5.4 to 6.2. Viscosity results obtained vis cosity values between 206.2 to 248 cps. Density measurement results were between 0.9839 to 0.9904 g/ml. The results of surface tension measurement results obtained between 35.83 to 37.10 dyne/cm. The results of particle size measurement results obtained from 23.83 to 30.43 nm. Conclusion: It can be concluded that the use of tween 80 and ethanol can form a microemulsion sys tem of eucalyptus oil, and with increasing concentrations of the combination of tween 80 and ethanol can increase the stability of the physical properties of the micro-emulsion system.