As one of the most cost-effective ways to improve energyefficiency, commissioning plays an important role in achievingbuilding energy savings goals. In this paper, a specific casestudy of energy saving-aimed commissioning was implementedin an existing building by using the methods of retrofittingand optimizing system control and operation. The casestudy site is a manufacturing plant with a gross area of1,212,940 sq. ft. (112,685 sq. meters). The work scope of thecase study covered the chilled water system with four 1200 ton(4,219 kW) chillers, heating system with two 70,000 lb/hr(31,751kg/hr) steam boilers, and thirty-four air handlingunits. After a detailed walk-through of the plant, a series ofsystem energy saving opportunities were identified for theexisting system operation such as chilled water bypassing thebuilding, blending of supply and return chilled water, highoxygen level in flue gas of the boiler, and extra fan powerconsumption for constant air volume systems and the like. Aftersystem retrofits and implementation of advanced control strategy,system overall performance was greatly improved. Thecompared utility data before and after the system optimizationindicate a total annual saving of $508,558 with the total electricityconsumption reduced by 15% and total gas consumptionreduced by 7%, considering the economic impact. In the paper,the system information, major retrofits, and advanced controlsequences and operational data were introduced anddiscussed.