Previous studies have proofed that the use of thermal energy storage can significantly improve the energy efficiency of a chiller plant via proper control. However, thermal energy storage systems are not commonly available in building cooling system. This study proposes an optimal control strategy to enhance chiller plant efficiency by utilizing the intrinsic thermal energy storage effect of the chilled water delivery system in a building. The fundamental idea of the proposed strategy is to maintain a high Part Load Ratio (PLR) of operating chillers so that they can produce cooling with high efficiency. The strategy also keeps the supply chilled water temperature within the allowed range, so that the building thermal comfort is not compromised. The proposed control strategy consists of two modes applicable for two different scenarios. Mode A is applicable during the end of office hours. It can bring forward the shutdown of chillers at the end of office hours and utilize the stored cooling energy to cool the building for a short period of time. Mode B is appliable for extreme low cooling load conditions (e.g., night mode operation), particularly when the actual cooling load is much lower than the rated capacity of a single chiller. The concept of the optimal control strategy has been validated on-site in a high-rise commercial building served by a large and complex cooling system whose chilled water delivery system has a thermal capacitance of around 3,312,600 kJ/K. On-site validation of Control Mode A shown the efficiency of chiller plant was improved by14% during the tested period, and the electricity consumption was reduced by 998 kWh. On-site validation of Control Mode B shown that the chiller efficiency could be improved by 43.3% during the tested hours and the electricity consumption was reduced by 2508 kWh.