Abstract: Objective To understand the concentration levels and distribution characteristics of fine particles (PM2.5) in air in Shijingshan district of Beijing. Methods Four surveillance sites were established, including 3 indoor sites and 1 outdoor site. The 24 h air concentrations of PM2.5 and PM10 at the four sites were consecutively detected with gravimetric method for 7 days in every months from May 2013 to April 2014. Results Overall, the median of indoor PM2.5 concentration was 61 g/m3, the PM2.5 concentration was higher in residence (69 g/m3) than in school (58 g/m3) and workplace (50 g/m3) (P 0.05). The difference between school and workplace had no statistical significance. The concentration of PM2.5 was lower in indoor site than in outdoor site (92 g/m3). According to national standard 3095-2012, the proportion of indoor air PM2.5 concentration above 75 g/m3 was significantly lower than that of outdoor site (41.7% vs. 60.7%, P 0.05). The indoor PM2.5 concentration was higher than indoor PM10 concentration (22.9%). During the surveillance period, 9 severe pollution events (PM2.5 150 g/m3) were detected, which lasted for 17 days cumulatively at the indoor sites (20.2%) and 24 days at the outdoor site (28.6%). The PM2.5 concentrations varied with season, which was highest in winter from January to March, and lowest in summer from July to August. The average ratio of indoor PM2.5/PM10 was 84.7%, higher than that of the outdoor ratio (64.1%). The ratio of I/O of indoor PM2.5 was 67.2%, higher than that of PM10 (53.9%). The indoor air PM2.5 concentration was positively correlated with outdoor air PM2.5 concentration. Conclusion The indoor air PM2.5 pollution was serious in the surveillance area in Beijing. The indoor air PM2.5 concentration was closely correlated with outdoor air PM2.5 concentration and also varied with seasons.