The purpose of this study is developing a formulation with an optimum sensory point by using yogurt added with rice flour and pregelatinized rice flour(alpha rice flour) optimized by response surface method(RSM). The pH, acidity, sugar content, viscosity and number of lactic acid bacteria during fermentation of two types of yogurt(added with rice flour and pregelatinized rice flour) optimized by RSM were analyzed As the fermentation time of both types of yogurt increased, pH showed decreasing trend. The titratable acidity showed increasing trend as fermentation time increased. Sugar content decreased as fermentation time increased. The reasons are believed to be the sugar decrease during glycolysis and lactic acid fermentation. Viscosity was the highest at 6 hours of fermentation. After 10 hours of fermentation, the viscosity was higher than it was before the fermentation. The number of lactic acid bacteria of yogurt added with rice flour and pregelatinized rice flour was 7.43~9.00 log CFU/mL, which is more than optimum value. Therefore, it is possible to confirm that adding rice flour during yogurt manufacturing increases the number of lactic acid bacteria.