The time series of indoor and outdoor particle number concentrations in a naturally ventilated residential building in Xi＇an were tested simultaneously for 7 days in summer. The relationships between indoor and outdoor concentrations were examined and discussed, and linear regression analysis was employed to correlate the indoor and outdoor concentrations. The diurnal cycles of indoor and outdoor particle concentrations of different particle size ranges all showed positive correlations between indoor and outdoor number concentrations, The I/O ratios of number concentrations varied with the increase of particle size in the range of 0.89 （±0.19） to 0.99 （±0.15）.
