Vegetation communities have been noted to affect soil organic carbon content via the addition of outer soil organic matter that in turn contribute to the formation of soil aggregates. The fractions of organic carbon in soil aggregates under different vegetation communities (forest and steppe zones) in the hill-gully region on the Loess Plateau were studied in this paper to explore the effect of different vegetation communities on soil structure. The study showed that: (1) Total soil organic carbon content in the forest zone of the study area was higher than that in the steppe zone. The order of total organic carbon for the plant communities in the forest zone was: Quercus liaotungensis community > Robinia pseudoacacia community > Sophora viciifolia community. Also the order of total organic carbon for the plant communities in the steppe zone was: Hippophae rhamnoides community > Lespedeza davurica + Artemisia giraldii community > Artemisia sacrorum + L. davurica community. (2) The proportions of active to total organic carbon and that of humus to total organic carbon were similar under each vegetation zone. However, the proportion of active to total organic carbon was greater than that of humus to total organic carbon under each vegetation community. (3) The >0.25 mm aggregates under the forest zone was significantly higher than that under the steppe zone. The pattern of the various forms of organic matter in the soil particles suggested initial increase in organic carbon, followed by decrease or flat curve with increasing aggregate size. A large proportion of soil organic carbon existed in 2~0.25 mm and <0.25 mm aggregates. (4) Active organic carbon content was significantly spatially different under the steppe zone. Each form of organic carbon in the Q. liaotungensis community was also significantly spatially different. The <0.25 mm aggregate humus was greater than other aggregates in the Q. liaotungensis community. (5) Unlike the other vegetation communities, there was no significant difference in terms of soil organic carbon between the 0~10 cm and 10~20 cm soil layers in the H. rhamnoides community.