Abstract: Mathematical model of ammonia synthesis converter with two bed radical flow in large scale synthesis ammonia plant has been presented, which describes the distributions of gaseous compositions, temperatures, and pressures along with the radius of the converter. Design optimization method of the converter that can produce 1kt ammonia a day has been proposed, by which the minimum catalyst volume of the converter can be obtained, when the operating pressure is 15.0, 10.0 and 7.5MPa respectively.