The indoor air quality is directly affected by particulate matters (PM). In this study the concentration of indoor and outdoor particles of an apartment and an office were measured. The effect of the concentration of outdoor particles on the concentration of indoor particles was analyzed. The results demonstrate that concentration of indoor and outdoor particles has 2-3 peak points during one day. The indoor to outdoor ratio increases as the size of particles decreases, which indicates that the free particles penetrate through the building envelopes more easily. The concentration of indoor particles is closely correlated with the concentration of outdoor particles, and the correlation increases as the size of particles decreases.