Applying biochar to soil has been suggested as a practice both to improve soil quality and to increase carbon sequestration of the soil. In this study effects of amendment of rice straw and straw biochar on organic carbon in aggregates various in fraction were investigated using the stable carbon isotope technique. 13C-labeled rice straw and straw biochar (prepared at 250℃ or 350℃) was mixed separately with red soil (Ferrosols) for incubation in the experiment. Results show that the effects of rice straw and biochar on fresh carbon and native soil organic carbon were completely different. After 112 days of incubation, the fresh carbon supplied by straw or biochar was mainly found in the 50 ~ 250 μm fraction of soil aggregates, making up 70.3% ~ 75.3% of the total. Compared with CK, amendment of rice straw significantly stimulated decomposition of the native organic carbon in the 250 ~ 2 000 μm fraction of soil aggregates, but had no such significant effect in the 50 ~ 250 μm and <50 μm fractions of aggregates, and the amendment of biochar (either SB250 or SB350) did not affect much decomposition of the native organic carbon in the 250 ~ 2 000 μm and 50 ~ 250 μm fractions of aggregates, but the amendment of biochar SB250 did significantly inhibit decomposition of the native organic carbon in the <50 μm fraction of aggregates, however, no such effect was found with the amendment of biochar SB350. The effects of rice straw and straw biochar on carbon in soil aggregates of the same size fraction differed mainly in distribution of fresh carbon, rather than native soil organic carbon.