Combing field investigation with laboratory analysis, we studied soil organic carbon(SOC) content in topsoil(0-20 cm) with different ages of the Stipa bungeana and Bothriochloa ischaemum grassland communities and presented the correlations of SOC content with geography, soil and vegetation characteristics of the communities in the loess hilly region, in order to proclaim soil carbon sequestration characteristics and notable influencing factors in the natural grassland restoration process. For the Stipa bungeana grassland, average SOC content dramaticlly increased from the 14 a to 18 a, with an average annual increment of 0.295 g/kg. The average SOC content gently increased first from the 18 a to 26 a, with an average annual increment of 0.186 g/kg, and then gradually decreased from the 26 to 45 a. The average SOC contents in 33 a and 45 a were 8.92% and 3.18% lower than that in the 25 a, respectively, but the difference was not significant. For the Bothriochloa ischaemum grassland, average SOC content gently increased from the 25 a to 40 a, with an average annual increment of 0.054 g/kg. The average SOC content obviously increased from the 40 a to 45 a, and the average SOC content in the 40 a was only 29.38% of that in the 45 a, and the difference was significant(p<0.05). SOC content in 0-10 cm soil layer was higher than that in 10-20 cm soil layer. Correlation analysis indicated that average SOC contents for Stipa bungeana and Bothriochloa ischaemum grasslands increased with the increases in restoration year, aboveground biomass, underground biomass and soil total nitrogen, and decreased with altitude and soil bulk density. Therefore, topsoils of Stipa bungeana and Bothriochloa ischaemum grasslands show the obvious function of carbon sequestration. Altitude, restoration year, aboveground biomass, underground biomass, soil total nitrogen and soil bulk density are the significant factors.