Ground-based observation of fine particulate matter (PM2.5) in Beijing was carried out continuously in 2006. The carbonaceous and ionic components, as well as their distribution characteristics and seasonal variation, were obtained. The annual mean mass concentration of PM2.5 was 176.6 ± 100.3 μg/m3. Long-range transport dust and local dust raised by strong wind during the spring made a considerable contribution to PM2.5 mass concentration. There was significant seasonal variation in carbonaceous and water-soluble ionic components associated with diverse emission sources, varying meteorological conditions during different seasons, and different mechanisms of formation for secondary aerosol ions. Comparing studies under different synoptic conditions suggested that PM2.5 pollution was mainly caused by transportation of particulates from remote sources, whereas hazy synoptic conditions are caused by local pollution. PM2.5 and visibility were negatively correlated, and the relationship between the concentrations of NH4+, SO42−, and NO3− with PM2.5 concentration during winter can be described using power function fitting.