Installing Natural Ventilation (NV) system in office buildings leads to the reduction of energy consumption of heating, ventilation and air conditioning (HVAC), which accounts for approximately 50% of total in an office building in Japan. However, it is difficult to estimate the NV performance before its completion, because the NV system is easily affected by the outdoor environment. Thus, its design method is not yet established. This study aims to easily estimate the NV performance by using transient airflow network calculation, and the wind pressure coefficient used in the method was obtained from Computational Fluid Dynamics (CFD). The theory of the model is described in detail by this study. Natural ventilation rate was calculated for two different models. Firstly, in order to learn about the availability of the calculation method, to know about available range of the time step and to calculate the transient conditions in a short period, a simple model, which consists of one node and two branches, was used. Secondly, in order to learn about the applicability of the method to actual buildings, an actual building was simulated as a complicated model, which consists of seven nodes and thirteen branches, and its natural ventilation rate was calculated by the method. As a result, at the simple model with the wind pressure coefficient that varies over time, the time-series change of the airflow rate was able to be obtained. In addition, at the actual building model, though its complexity, the time-series change of temperature and ventilation flow rate was obtained by the method. It was shown that the temperature changes are slower than that of pressure, because of the heat capacity of the air, which correspond the real phenomenon.