As increasing demand for energy efficient buildings with better experience of user comfort, the built environment industry is faced with the challenge to ensure that the building performance prediction, especially energy performance, during the design stage is achieved when a building inuse stage. There are, however, significant evidence suggested that buildings do not perform as anticipated at the design stage. This discrepancy is commonly referred to as the 'Energy Performance Gap'. Building audit and Post Occupancy Evaluation (POE) is one of the efficient processes to minimize energy waste, improve indoor environmental quality and reducing performance gap by observing, surveying, monitoring, and documenting building's systems and in-use indoor condition. Especially with the in-use building, POE is a user-oriented process to improve an existing building's indoor environmental quality and operating performance. In this study, a case study of an office building's energy audit, POE and dynamic simulation were carried out. The building energy audit data and POE study results were analyzed in order to identify factors of the energy performance gap. The building dynamic simulation was used to identify the utility of the strategy for reducing the energy performance gap. Furthermore, the building energy audit data and POE study was applied for evaluating and calibrating dynamic simulation model. Through this paper, it shows that building energy audit and POE study is possible to reduce energy performance gap, and to increase the user comfort with indoor environmental quality. The research findings were discussed in this paper.