Abstract: The carbon content in PM_2.5 is measured by Aethalometer (for BC) and Sunset OC/EC analyzer (TOT, for EC), respectively, during the winter of 2004 in Beijing. The experimental result shows that black carbon and elemental carbon have a good agreement with a high correlation coefficient (R²=0.80), so does organic carbon and elemental carbon (R²=0.82). The ratio of OC to EC is higher in the daytime than at night and higher at normal day than at temperature inversion day. In addition, there is no significant difference of OC to EC ratio between day and night at temperature inversion day. The average ratio of OC to EC is 4.76. It is also proved that ratio of OC to EC larger than 2 is not the exact index for secondary pollution.