According to the Green Building Labeling System in Taiwan, energy saving is the most important factor among the four aspects of evaluations including ecology, energy saving, healthy and waste reduction. This indicates the importance of indoor environment. Taiwan is located at subtropical climate. There are approximately 4 months of a year when the outdoor temperature is lower than the indoor temperature. This advantage can be used to adapt the thermal comfort indoors and improve the indoor quality. For the building without air conditioning, natural ventilation is very good strategy. If natural ventilation strategy can be properly designed, it can not only decrease the dependence of air conditioning but also reduce the energy use. In order to correctly design naturally ventilated building, design tool is needed for instance wind tunnel, math model, computational fluid dynamics (CFD)…etc. Compared to others, CFD is the tool commonly used in many fields. The reason why CFD is popular is the rapid development of computer engineering. The use of CFD technology can predict the air flow pattern, temperature stratification…etc. Thus, due to the CFD calculation, the opening size and location can be determined for increasing the comfort condition of a building. Data obtained from the CFD can be referred at the design stage. The investigation mainly aims to analyse the indoor physical environment by means of CFD in order to determine the passive design strategy. General findings of the study show that high wind speed can provide better air change rate whatever the indoor flux and air opening conditions. The high air change rate can efficiently reduce the indoor temperature and improve the indoor environment. Under the same wind speed, the air change rate is depended on the heat flux of a room which depicts that buoyancy ventilation can be a useful strategy.