The high lactose content of cheese whey adversly affects the activities of the methane archaea due to the rapid pH drop during directly treatment in the UASB reactor and cheese whey pH must be controlled first in pre-treatment tank and it would be fed to the UASB reactor at stable pH. In this study, the effects of temperature, pH and dilution ratios on lactose conversion were modeled in the pretreatment tank depending on the data obtained in the experimental conditions determined by the statistical experimental design method. In this study, lactose conversions determined by measuring lactose concentrations with DNS method at the conditions determined by the Box-Wilson experimental design method, and they were studied in 20 reactors. The obtained data was evaluated by Design Expert 7.0.0 Trial "package program. Using the obtained data in the "Design Expert 7.0.0 Trial" package program, the following model equation was obtained in the pre-treatment tank: Y1 = 44,70 + 14,11*X1 + 1,35*X2 +2,85*X3 – 0,29*X1*X2 + 0,80*X1X3 – 0,13*X2X3  + 6,81*X12 + 3,24*X22 + 1,75*X32(Y1 = % Lactose Conversion, X1 = Temperature, X2 = pH, X3 = Dilution Ratio) In the pretreatment reactor, a model equation that gives the changes of the lactose conversion with temperature, pH and dilution ratio was developed and controlled feed conditions for the UASB reactor were determined. Key words: Cheese whey, lactose conversion, pre-treatment reactor, UASB reactor, model equation.