Two different pollution situations of O₃ and PM₂.₅ during summer in Beijing were analyzed from theperspective of synoptic situations, meteorological elements, precursors, atmospheric oxidation, back-trajectories of air mass and chemical compositions of PM₂.₅. The results showed that the synoptic situations in the pollution situation that O₃ reached middle level pollution and PM₂.₅ maintained low concentrations (O₃high-PM₂.₅ low) could be characterized as northwest gas flow in 500 hPa height and high-pressure rear in the ground. Whereas the synoptic situations in the pollution situation that O₃ and PM₂.₅ both reached middle level pollution (O₃-PM₂.₅ high) could be characterized as westerly gas flow in 500 hPa height and low pressure in the ground. Compared with the O₃high-PM₂.₅ low situation, meteorological elements in O₃-PM₂.₅ high situation could be characterized as stronger southerly winds and higher relative humidity. In the O₃-PM₂.₅ high situation, initial concentrations of O₃ and PM₂.₅ were higher and diurnal variations of PM₂.₅ were more significant, nevertheless, the average concentrations of O₃ were lower than those in the O₃high-PM₂.₅ low situation, respectively. The analysis of precursors, atmospheric oxidation and chemical compositions of PM₂.₅ showed that the accumulation and hygroscopic growth of PM₂.₅ under unfavorable meteorological conditions as well as the regional transport caused by strong southerly winds might be the main factors leading to high PM₂.₅ concentrations in O₃-PM₂.₅ high situation.