Abstract: In order to study the biological oxidative stress effect of humic-like substances (HULIS) in fine particulate matter (PM_2.5), the concentrations of carbonaceous components and metal elements in atmospheric PM_2.5 in the urban and suburb areas of Beijing were measured. Rat alveolar macrophages (NR8383) were used as cell model to determinate the ability of PM_2.5 and its important components to induce endogenous ROS and the contribution of HULIS in PM_2.5 to the induction of endogenous ROS. The results showed that: (1) The concentrations of carbonaceous components and endogenous ROS (the unit is μg zymosan/m³, representing the concentration of endogenous ROS per unit volume of air) in PM_2.5 in winter were significantly higher than those in summer, but PM_2.5 in summer had higher biological oxidative potential (the unit is μg zymosan/mg PM, representing the biooxidative potential of the unit mass of particulate matter). (2) The concentrations of carbonaceous components and some metal elements in PM_2.5 all had high correlation with the concentration of endogenous ROS induced by fine particulate matter. (3) The endogenous ROS induced by HULIS alone was also higher in winter ((55.46±22.69) (43.27±23.89) μg zymosan/m³ in urban and suburbs, respectively) than in summer ((18.73±11.94) (7.88±3.63) μg zymosan/m³ in urban and suburbs, respectively). There was also a good correlation between the endogenous ROS concentration induced by HULIS alone and the total endogenous ROS concentration induced by PM_2.5 at the two sampling sites in the urban and suburban areas of Beijing. The results showed that HULIS was a key component that induced ROS generation in PM_2.5 at the two sampling sites in the urban and suburban areas of Beijing, and the contribution rates of HULIS to PM_2.5-induced endogenous ROS were 42% and 37%, respectively.