The critical behavior of pseudobinary microemulsion systems {water/sodium di(2-ethylhexyl) sulfosuccinate (AOT)/n-decane} with various droplet sizes was studied by measurements of refractive index.It was found that the critical exponents β for all systems approach 0.327 in a region sufficiently close to the critical temperature, which is consistent with 3D-Ising universality class. The critical temperatures linearly decrease as the dispersed droplet sizes increase. The critical amplitude almost linearly increases with increasing the dispersed droplet sizes. microemulsion, critical behavior, critical exponent, critical amplitude, AOT Microemulsion (nanoemulsion) is a colloidal dispersion system consisting of discrete droplets with a size range of 10-200 nm dispersed in another continuous liquid phase. It is usually water, surfactant and oil mixture and has some interesting physical and chemical properties, such as optical transparence, thermodynamic stability, high solubility, low viscosity and low interfacial tension. Microemulsion is extensively applied to industry and agriculture, pharmacy, food technology, nanomaterial synthesis, biotechnology and oil recovery due to its special structures and properties, and the phase equilibrium and the critical phenomena of microemulsion system have aroused our attention. At present, some experimental controversies about whether microemulsion system belongs to 3D-Ising model or Fisher normalization model remain unsolved [1,2] . The critical behavior for microemulsion of {water/sodium di(2-ethylhexyl) sulfosuccinnate (AOT)/n-alkane} with a constant molar ratio of water to AOT (ω =40.8) in our previous works [3][4][5] has been studied and the results have shown that the critical behavior of the microemulsion system may be described as 3D-Ising characteristics in a region sufficiently close to the critical temperature. These results were confirmed by other experiments [6] . However, the effects of the various molar ratios ω in the microemulsion systems on the critical temperature T c , the critical exponent β, and the critical amplitude B are not reported. Three-component mixtures of water, AOT and n-decane can form water-in-oil microemulsion of well-defined droplet size determined by fixing the molar ratio ω of water to AOT, and the droplet sizes are directly proportional to ω. For these mixtures, a lower consolute critical point was observed. Above the critical temperature T c , the mixture is separated into two microemulsion phases of different composition but with the same dispersed droplet sizes [6,7] . Therefore, such a microemulsion system can be regarded as a pseudobinary mixture. The critical behavior of five pseudobinary microemulsion systems with various sizes of droplets (the range of ω was selected as 25-50 in the experiment) was studied by the measurement of refractive index. The experimental results were analyzed, clarifying