Adverse health effects by ambient fine particulate matter (PM) have been suggested to be associated with production of reactive oxygen species (ROS), such as hydroxyl radical (·OH).However, very few studies have quantified ·OH production from PM2.5 in Beijing which is one of the most polluted cities in the world.Ambient PM2.5 samples were collected on 47 mm Teflon filters and Quartz filters using a four-channel low-volume sampler on the campus of Peking University in June 2014 in Beijing.The production of·OH induced by PM2.5 was measured in an aqueous buffered extract solution as a surrogate lung fluid (SLF).Organic carbon (OC),elemental carbon (EC), water soluble inorganic ions and transition metals have also been analyzed.Preliminary result shows that production of·OH from PM2.5 extract has a linear and positive correlation with extraction time.During our sampling period,the amount of·OH production from PM2.5 was 2.8±1.2 ng/μg after 2 h extraction time,which was much higher than that in Los Angeles in U.S.Transition metals show good correlation with.OH, which indicates that transition metals play a dominant role in · OH generation.More detailed information about chemical composition, sources and ROS generation of PM2.5 will be discussed.