The ecological consequences of grassland afforestation in arid/semiarid sandy regions are not well know wilh respect to tree species and stand age. The present study quantifies the changes in above and belowground carbon (C) stocks following afforestation in the southeastern keerqin sandy land with species of Mongolions pine and poplar .we studied 15-, 14-, and 30-year-old Mongolions pine plantation , 7-, 11-, and 15 -year-old poplar plantation, andadjacent grassland. The results show that total ecosystem C stocks increased follwing grassland afforestation. Aboveground C stocks increased at a rate of 2.75 Mg/ha-1yr-1 in the poplar plantation, and 1.06 Mg/ha-1yr-1 in the Mongolions pine. Mineral soil C stocks decreased during the early stage of forest astablishment, but recovered with increasing stang age. Root C stocks increased significantly in the Mongolions pine plantation ,but the poplar plantation showed no such increased relative to the grassland .our results indicate that afforestation of the grassland in the southeastern keerqin sandy lands would sequester more C than would continuous grassland. Tree species selection and stand developmental age should be considered in planning future afforestation projects