Natural ventilation is driven by either buoyancy forces or wind pressure forces or their combinations that inherit stochastic variation into ventilation rates.Since the ventilation rate is a nonlinear function of multiple variable factors including wind speed,wind direction,internal heat source and building structural thermal mass,the conventional methods for quantifying ventilation rate simply using dominant wind direction and average wind speed may not accurately describe the characteristic performance of natural ventilation.From a new point of view,the natural ventilation performance of a single room building under fluctuating wind speed condition using the Monte-Carlo simulation approach was investigated by incorporating building facade thermal mass effect.Given a same hourly turbulence intensity distribution,the wind speeds with 1 min frequency fluctuations were generated using a stochastic model,the modified GARCH model.Comparisons of natural ventilation profiles,effective ventilation rates,and air conditioning electricity use for a three-month period show statistically significant differences(for 80％ confidence interval)between the new calculations and the traditional methods based on hourly average wind speed.