Adverse health effects of ambient PM2.5 ( dp < 2.5 μm) can be associated with the production of reactive oxygen species (ROS), among which hydroxyl radical (•OH) is the most reactive. However, •OH generated by PM2.5 has not been quantified and studied in the North China Plain (NCP), which has suffered from heavy air pollution in recent years. In this study, PM2.5 samples were collected at an urban site (Beijing) and a suburban site (Wangdu), extracted in a cell-free surrogate lung fluid (SLF), and •OH generated in the extracts were quantified. The results show that more •OH is produced on heavily polluted days than that on clean days (2.0 and 1.6 times higher in Beijing and Wangdu, respectively). The production of •OH per unit mass (ng/μg·PM2.5) decreases with the increase of ambient PM2.5 concentration because SO42-, NO3-, and NH4+ dominate the increased PM2.5, while these secondary inorganic components do not contribute to the generation of •OH. Trace metals (e.g., Fe, Cu, Se) and carbonaceous species (organic carbon and elemental carbon) correlate well with the •OH production, indicating that particles from combustion sources including coal combustion, vehicle exhaust, and industry contribute more to •OH generation.