The effect of adding Saccharomyces boulardii on yogurt quality was studied. Yogurt control was made using whole cow’s milk and classic starter culture. Other three treatments of yogurt were made by adding 1%,2% and 3% of Saccharomyces boulardii with yogurt starter. pH values and proteolytic activity of all yogurt treatments were determined during fermentation time. Changes in physicochemical and microbial properties of yogurt product were observed during storage time (21 days at 4°C). Yogurt samples with added yeast to starter cultures showed a slight increase in pH values during the 6 hours of fermentation. After fermentation time, pH and proteolytic activity of yogurt with 3% yeast were 4.05 and 250 μg/ml while control sample was 4.22 and 200 μg/ml respectively. pH, TN, WSN, TVFA and WHC values of yogurt with Saccharomyces boulardii were slightly increased whereas decreased the STS percentage compare with control yogurt without yeast during storage time. The addition of Saccharomyces boulardii improved the survivability of bacterial starter culture. After 21 day, Saccharomyces boulardii counts were 5.78, 6.01 and 6.31 Log. CFU/gm for yogurt with 1%,2% and 3% yeast respectively whereas Log. lactic acid bacteria of yogurt with 3% yeast was 7.53 and 7.55 for Lactobacillus bulgaricus and Streptococcus thermophilus.