Abstract: To understand the mass size distribution characteristics and its sources of carbonaceous aerosol, the particle samples were collected by an Andersen cascade impactor in Tianjin City from 2009to 2010. Organic carbon (OC) and elemental carbon (EC) in atmospheric particles were analyzed with the DRI carbon analyzer. The results showed that both EC and OC were mainly enriched in fine particles (≤2.1μm), the annual mass concentration of EC and OC in PM2.1 were (2.6±0.9) and (21.5±7.7)μg/m^3with proportions accounting for 72% and 60% of PM₉, respectively. Significant seasonal differences were found during the sampling period. The seasonal average concentration of EC in spring, summer, autumn, and winter were (1.7±0.3), (2.1±0.4), (3.1±0.5) and (3.7±0.5)μg/m³, while it were (17.6±0.4), (14.4±1.1), (21.9±1.8) and (32.1±2.5)μg/m^3for OC respectively during the four seasons. Log-normal size distribution of EC showed a tri-modal with peaks appeared in sizes of ≤0.43, >0.65-1.1and >4.7-5.8μm, the highest concentration was exited in size of ≤0.43μm. OC showed a bi-modal size distribution with peaks appeared in sizes of >0.65-1.1and >4.7-5.8μm, with the highest concentration usually exited in the size of >0.65-1.1μm. Ratios of OC and EC indicated that the main origins of carbonaceous aerosols in fine particles were attributed to automobile exhaust fumes, coal combustion and cooking combustion emissions, while the road dust and the coal combustion were the main sources of coarse particles.