Effects of grassland species identity and cultivation time on soil organic carbon were studied in cultivated soil in Lhasa National Ecological Research Station, Chinese Academy of Sciences in Tibet, China. Soil samples were collected from one type of local original montane shrub soil (control) and five types of cultivated soil with different grassland species and cultivation time, i.e. one type of Dactylis glomerata L. soil having been cultivated for 10 years (D10), two types of Elymus nutans Griseb. soil having been cultivated for 4 and 10 years (E4 and E10), and two types of Medicago sativa Linn. soil having been cultivated for 3 and 10 years (M3 and M10). Compared with the control (the montane shrub soil), D10 significantly increased total organic carbon (TOC) in soil at both 0-5 cm and 10-30 cm, but did not significantly affect it in soil at 5-10 cm. E10 significantly decreased TOC in soil at 0-5 cm, and M10 significantly increased TOC in soil at both 0-5 cm and 10-20 cm. All these differences in TOC resulted from different responses of organic carbon in soil fractions to species identity and cultivation time. In D10, particulate organic carbon (POC) of coarse sand (250-2000 μm) decreased and mineral-associated organic carbon (MOC) of silt and clay (< 50 μm) increased in soil depths of 0-5 cm and 10-30 cm compared to the control. E10 significantly reduced POC of sand (50-2000 μm) and MOC of coarse clay (0.2-2 μm) in soil depth of 0-5 cm, but increased MOC of fine clay (< 0.2 μm) at the same soil depth. In M10, POC of coarse sand (250-2000 μm) decreased and MOC of silt (2-50 μm) increased in soil depth of 0-5 cm. In addition, in soil of 10-20 cm in M10, POC of coarse sand (250-2000 μm) decreased, but POC of fine sand (50-250 μm), MOC of silt (2-50 μm) and MOC of fine clay (<0.2 μm) increased. Compared to E4, E10 significantly decreased TOC in soil of 10-20 cm. And E10 reduced POC of coarse sand (250-2000 μm) and increased POC of fine sand (50-250 μm) and MOC (< 50 μm) compared to E4. Compared to M3, M10 significantly decreased TOC in soil of 5-30 cm. Although M10 significantly increased POC of fine sand (50-250 μm) and MOC of coarse clay (0.2-2 μm) in soil of 0-5 cm, it significantly reduced MOC of fine clay (<0.2 μm). In soil of 5-10 cm in M10, compared to M3, POC of sand (50-2000 μm) significantly decreased and MOC of coarse clay (0.2-2 μm) increased. M10 significantly increased MOC of coarse clay (0.2-2 μm) in soil of 10-20 cm, but decreased POC of coarse sand (250-2000 μm) and MOC of fine clay (< 0.2 μm). Meanwhile, significant decrease in POC of sand (50-2000 μm), MOC of silt (2-50 μm) and fine clay (< 0.2 μm) were observed in soil depth of 20-30 cm in M10. The results indicated that short cultivation time were beneficial to the enhancement of soil organic carbon sequestration.