A utility plant is one of key unit plants in large scale refinery and petrochemical plants that supplies the steam and the electricity using fuel oil and gases. As the energy cost and the concern on the environmental pollution have been increasing, the energy saving and the operation optimization of the utility plant have received attention from both academia and industry. This paper proposes a hierarchical optimization method for the utility plant optimization that consists of top and bottom level optimization. At the upper level, the total utility plant including electricity supply and demand is optimized. At the lower level, the solution for the steam generation obtained from the top level is refined to achieve the optimal load allocation for the boilers. Various unit process models have been developed from the operation data and the material and energy balances. The key variables that mainly determine the operation cost for the utility plant have been selected and optimized. The optimization result has shown that the operation cost was reduced by 6.2% and the average efficiency of the boilers was improved by 0.3 to 1.7% through optimal boiler load allocation.