The ambient air quality in Beijing was comprehensively assessed based on the real-time pollutant concentrations monitored at urban, suburban, roadside, and background sites in 2013. The results showed that the annual average concentration for CO, NO2, SO2, O3, PM2.5 and PM10 in 2013 was 2.0 mg/m3, 55.6 µg m–3, 28.5 µg m–3, 48.0 µg m–3, 92.2 µg m–3 and 118.6 µg m–3, respectively. The annual average concentration of CO, NO2, SO2, PM2.5 and PM10 was highest in roadside, while that of O3 was highest in background stations. The mean monthly statistics indicated the maximum concentration of CO, NO2, SO2, PM2.5 and PM10 occurred in January because of larger emissions in heating season, lower wind speed and higher relative humidity (RH), while the minimum was found in July or August due to larger precipitation or photochemical degradation. The peak concentrations of O3 occurred during May to August due to higher temperature and solar radiation which could promote the photochemistry activity. The monthly variation is also reflected in the corresponding season. Diurnally analysis showed the CO, NO2, SO2, PM2.5 and PM10 in urban and roadside area had two increase phases accompanying with the traffic peaks. Beside the temporal variation, we also found the spatial variation that higher concentrations of O3 and other pollutants occurred in northern and southern districts/counties, respectively. It could be attributed to the spatial distribution of various pollutant emissions in Beijing and the impact of pollutant transport from neighboring provinces. Moreover, we examined the visibility in Beijing and found its significant correlation with PM2.5 concentration and RH, respectively. Lastly, the air quality in Beijing was compared with that in other mega cities in the world. The higher pollutant concentrations and PM2.5/PM10 ratio indicated that the mitigation of the air pollution especially the PM2.5 pollution in Beijing still had a long way to go.