Abstract: In order to understand the indoor and outdoor pollution situation of high-rise apartment buildings, the indoor and outdoor particulate matter concentrations of one apartment building nearby the road in Xi’an city under natural ventilation were simultaneously tested. The results show that the indoor and outdoor particulate matter pollution is serious and fine particles accounts for the main part of inhalable particles. The changes of indoor and outdoor concentrations present good consistency, the fluctuations of percentage of fine particles in outdoor PM₁₀ are greater than indoor, and the particulate matter concentrations along the height direction of the building exhibit bimodal distribution. In the absence of clear indoor pollution sources, the I/O ratio of PM_2.5 and PM₁₀ is usually less than 1, while the I/O ratio of PM_2.5 and PM₁₀ is greater than 1 if there are obvious pollution sources in indoor. The I/O ratio decreases as the size of particles increases. There is a certain linear relationship between the concentrations of indoor and outdoor particles and, for high-rise apartment buildings near the road, the indoor PM_2.5 concentration is effected by the outdoor concentration more than PM_1.0 and PM₁₀.