The purpose of this study was to investigate the day-night variation of carbonaceous aerosols in PM10 during spring and winter over Xi'an. PM10 samples were collected during 19 Dec 2006 to 21 Jan 2007 (Winter) and 1 Apr 2007 to 30 Apr 2007 (Spring). Organic carbon (OC) and elemental carbon (EC) concentrations were measured using thermal-optical method. PM10, OC, and EC concentrations in winter were 455.0, 62.4, and 7.5 microg/m3 during daytime, and 448.7, 66.1, and 6.9 microg/m3 for nighttime, respectively, while in spring were 397.9, 26.7, and 6.9 microg/m3 for daytime, and 362.1, 31.9, and 8.6 microg/m3 for nighttime. The correlation coefficient of OC and EC during daytime in winter was 0.44, while a strong relationship between OC and EC was observed in spring daytime, indicating that the emission sources of carbonaceous aerosol in winter were more complicated than those in spring. Due to high OC levels and the weaker dispersal ability of atmosphere, the mean concentrations of SOC during winter (8.9 and 10.2 microg/m3 at daytime and nighttime) were much higher than those in spring (2.8 and 3.4 microg/m3). Factor analysis on the eight carbon fraction indicated that coal combustion and biomass burning were the major sources for carbonaceous aerosol in winter, while vehicle exhaust played an important role in spring.