Dahi is a traditional Indian fermented milk product that is similar to plain yogurt in appearance and consistency. As compared to yogurt, dahi has lower acidity, consequently it seems to be more effective in production of probiotic or synbiotic foods. The aim of this study was optimization of the production process and evaluation of physicochemical and microbial properties of synbiotic dahi by response surface methodology (RSM) based on Box Benken Design. Inulin (0-2.5%), tragacanth gum (0-0.06%) and storage time (1-19 days) were the factors that their impacts on the dependent variables were evaluated. In order to optimize variables, three traits including firmness, syneresis and viability of probiotic Lactobacillus acidophilus (LA₅) were considered as responses. Finally, the appropriate power and fit of model were indicated based on determination coefficient (R²), adjusted determination coefficient and coefficient of variation (CV). The determination coefficient (R²) for firmness, syneresis and microbial counting in synbiotic dahi was obtained 0.99, 0.91, and 0.98, respectively. The increase in the concentration of inulin and storage time led to improve of syneresis and firmness of the products. Also, the increase in concentration of tragacanth up to 0.03% increased firmness, while afterward up to 0.06% concentration caused negative effect and reduced firmness. The increase in concentration of tragacanth and inulin had positive effect on probiotic counting of the products, while storage time had deleterious effect on it. According to the results of optimization experiments, optimal conditions were 2.5% concentration of inulin with 0.03% concentration of tragacanth gum in 18-day holding period.