Evaporation phenomenon of sessile droplet plays an important part for inkjet technology. In this study, vapor concentration evaporated from ethanol-water mixture droplet was investigated from both experiment and simulation. Z type Schlieren visualization together with Abel inverse transform was used to obtain the vapor mole fraction from the experiment. Experiment was conducted inside the chamber in which the relative humidity was controlled at 75%RH by saturated salt solution technique. Ethanol-water mixture droplet was prepared at the ethanol concentration of 100% (pure ethanol) and 75% by volume fraction. Simulation was performed with modified solver of in-house OpenFoam source code based in scalarFoam. Simulation was firstly performed assuming ideal liquid with constant liquid mole fraction. Then, activity coefficient was applied together with the consideration of liquid surface concentration change due to the evaporation. Experiment and simulation results were compared. Consistency between both results was observed in pure ethanol case even in the condition of ideal liquid assumption. For ethanol-mixture droplet, deviation between experiment and simulation result was found with the condition of ideal liquid assumption. When activity coefficient and variable liquid surface concentration condition were applied, consistency was found at the beginning stage of the evaporation (t = 0 s).