Daily 24-hour PM2.5 samples were collected continuously from January 1 to December 31, 2010. Elemental concentrations from Al to Pb were obtained using particle induced X-ray emission (PIXE) method. This was the first full year continuous daily PM2.5 elemental composition dataset in Beijing. Source apportionment analysis was conducted on this dataset using the positive matrix factorization method. Seven sources and their contributions to the total PM2.5 mass were identified and quantified. These include secondary sulphur– 13.8 μg/m3, 26.5%; vehicle exhaust– 8.9 μg/m3, 17.1%; fossil fuel combustion– 8.3 μg/m3, 16%; road dust– 6.6 μg/m3, 12.7%; biomass burning– 5.8 μg/m3, 11.2%; soil dust– 5.4 μg/m3, 10.4%; and metal processing– 3.1 μg/m3, 6.0%. Fugitive dusts (including soil dust and road dust) showed the highest contribution of 20.7 μg/m3 in the spring, doubling those in other seasons. On the contrary, contributions of the combustion source types (including biomass burning and fossil fuel combustion) were significantly higher in the fall (14.2 μg/m3) and in the winter (24.5 μg/m3) compared to those in the spring and summer (9.6 and 8.0 μg/m3, respectively). Secondary sulphur contributed the most in the summer while vehicle exhaust and metal processing sources did not show any clear seasonal pattern. The different seasonal highs and lows from different sources compensated each other. This explains the very small seasonal variations (< 20%) in the total PM2.5.