Ambient particulate n-alkanes were determined for fine particle (PM2.5) samples collected from Sep 2003 to July 2004 in Beijing,China.The average concentration of total n-alkanes (Σn-alkanes) from C11 to C34 was 425.72 ng/m3,ranged from 7.02 to 2893.28 ng/m3.The concentration distributions of n-alkanes homologues in this study exhibited peaks at C21 and C29 in beating season,and C29 in non-heating season.The average carbon preference index (CPI) value was 1.88 in the range of 1.18-3.88.The maximum CPI in summer indicated the contribution of biogenic origins such as plant wax;while the minimum CPI value in winter was probably a result of fossil fuel combustion.Preliminary estimation from these results showed that 59% of the n-alkanes in PM2.5 in Beijing summer originated from plant wax,while 74%-88% was from fossil fuel combustion in other three seasons.Source estimation was further performed using principal component analysis method.Two major components were yielded accounting for 57.3% and 30.9% of the total variance,which presented the fossil fuel and biogenic contribution,respectively.