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In the loess hilly region of China, a large amount of arable land has been replaced by artificial forests, resulting in increased storage of soil organic carbon (SOC). To elucidate the process and mechanisms of SOC sequestration following conversion of cropland to forest, the storage and sequestration rates of SOC associated with sand, silt and clay was measured by physical fractionation on Robinia (Robinia pseudoacacia L.) and Caragana (Caragana korshinskii) lands that have been stood on former cropland for 15~45a. Compared with cropland, the content of carbon in all soil particle-size fractions on all afforested lands was significantly increased with the stand time. Moreover, the highest increase of carbon was in the top soil layer (0~10cm) and similar increase amplitude of carbon was found in 10~60cm soil layers. During the 15~45a afforestation on former cropland, the highest annual sequestration rates of carbon stock in 0~20cm soil depths was in silt, which were 0.21Mg/(hm·a) and 0.11Mg/(hm·a), and similar sequestration rate of carbon was in sand with average value of 0.13Mg/(hm·a) and in clay was 0.06Mg/(hm·a) on Robinia and Caragana lands, respectively. The same changing trend also occurred in 0~60cm soil depth, and sequestration rate of carbon associated with all mineral particles was 1.6 to 2.5 times higher than those in 0~20cm soil depth. Following the annual increase rates, the carbon stocks in sand, silt and clay were varied by 2.6, 1.1 and 0.8 times on Caragana land, and those were varied by 8.3, 2.2, 2.8 times on Robinia land respectively after 45 years conversion of cropland to forest. The average percentage that contributed to accumulation of total SOC in different mineral particles was in the order: silt carbon (51%) > clay carbon (26%) = sand carbon (23%). Additionally, there was a more significant linear correlation between SOC content and carbon management index (CMI) than the linear correlation between SOC content and activity of carbon pool (A). In conclusion, long term stand Robinia on former cropland sequestrated more SOC than stand Robinia in the area. However, carbon in slit was the main fraction for carbon sequestration and carbon in sand showed the fastest turnover rate on both afforested lands.