Since 1980, China has launched largescale afforestation campaigns with an area of 30 million hectares. Even if afforestation only slightly affects ecosystem C stocks at the local level, it could have a significant effect on the global C budget if enough agricultural land is converted. We studied the change of C stock in biomass, forest floor and soil and C circle after the cropland was converted to larch (Olga Bay Larch, Larix olgensis Henry) forest in seven stands in afforestation chronosequence, and made comparisons of ecosystem carbon change with adjacent Korean pine (Pinus koraiensis Sieb. et Zucc), pasture, and cropland. An adjacent 250yearold natural forest was included as a control for the possible longterm carbon storage in Changbai Mountain, northeast China. The findings are that: the C stocks in biomass and forest floor both increased over stand age; from the third year to the 33rd year of croplandtoforest conversion program, the C stock changed from 4.134 to 74.11 Mg/hm2 for the biomass, and from 0 to 11.31 Mg/hm2 for the forest floor. Soil carbon density initially decreased during the first 12 years to a minimum level of 75.87 Mg/hm2. Then it gradually recovered and started to accumulate and reached a level of 84.28 Mg/hm2 in the 21st year, as high as that of cropland. The ecosystem C stocks were 81.778, 114.488, 130.004, 187.255 and 178.580 Mg/hm2 after afforestation of 3、12、22、33 years for larch and 33 years for Korean pine. The carbon sequestration of larch increased over afforestation time. There was no significant difference in the ecosystem C stocks between larch and Korean pine. The high ecosystem carbon contents (269.57 Mg/hm2) in the 250yearold afforested stand suggest that the afforestation ecosystem in Changbai Mountain area may serve as potential sinks for C in the long run.