Abstarct: In the present work, we investigated and optimized the digestion efficiency, A/S and N/S of mixed bauxite in Jajarm Alumina Factory. Factory feed is a mixture of bauxites of Jajarm (35 %), Tash (50 %), Yazd (10 %) and Shirin Cheshmeh (5 %). To achieve optimum process conditions, effective parameters on digestion such as Na2O concentration of alkaline solution, temperature, amount of lime addition and retention time were studied. Before digestion experiments on mixing bauxite, the digestion conditions of each bauxite were studied individually according to the current conditions of the process. baxsed on the results, the optimum amount of lime for the Jajarm, Tash, Yazd and Shirin Cheshmeh was 10, 6, 6 and 12 %, respectively, the temperature digestion of Yazd bauxite is 275 oC, which is 5 oC more than other bauxites. A 4-factor 3-level response surface methodology was applied for the design and analysis of experiment with the optimization of effective parameters on digestion. Two quadratics and one linear model were derived for the prediction of digestion efficiency, A/S and N/S responses. The results showed that the optimum amounts for Na2O concentration, temperature, amount of lime addition and retention time were 180 g/l, 275 ° C, 7.73% and 50 minutes, respectively in which the digestion efficiency, A/S and N/S reached 72.05%, 1.169 and 0.27 respectively. Validation experiment showed that the digestion efficiency, A/S and N/S were 72.22%, 1.162 and 0.28% respectively which means 2% increase in digestion efficiency and a 0.09 and 0.02 decrease in A/S and N/S respectively compared to the current operating conditions.