This study is to optimize design parameters for waste heat recovery boiler with water tube and three drum. The design optimization considered here is to find the most economic dimension of the boiler considered which results in a minimum cost in producing a unit amount of steam per given period of operation. For this purpose, optimize design have to determine what are the main parameters of affecting the total cost of producing a unit amount of steam which is comprised of manufacturing cost of the boiler, operating cost of the fan etc.