Heating, ventilating and air-conditioning (HVAC) systems in commercial buildings consume the largest amount of energy usage. Recent surge of energy cost necessitates constant re-evaluation of HVAC system for most of buildings. The objective of this study is to present the strategic approach on energy saving analysis of the HVAC system and chiller sizing optimization for a library building. Energy simulation tool for buildings (eQUEST) has been applied to verify and predict the long term energy consumption for the HVAC systems. Field measurements of power consumption from building energy management system (BEMS) of overall HVAC system have been conducted comprehensively for one year to quantify energy saving results. The results from energy simulation revealed good agreement for energy simulation and field measurement data with the error form 0.87 % to 8.77%. Besides, the optimization sizing of chillers system and energy savings could be achieved satisfactorily by utilizing energy modeling using the actual chiller performance curve. By implementing the energy modeling for chiller sizing based on cooling load profile, the energy saving for HVAC system can be obtained satisfactorily at the saving of 110362 kWh per year. It is expected that the study will stimulate a more robust investigation of energyefficient and cost-effective HVAC system specific for the library buildings.