In this study, effects of five-year no-tillage and straw returning on composition and organic carbon content of soil aggregates in farmland fluvo-aquic soil were investigated in a long-term experiment site of conservation tillage. The research results showed that: compared with the conventional tillage treatment, content of >5 mm mechanically stable aggregates under conventional tillage+straw returning, no-tillage, no-tillage+straw returning treatments were significantly improved, the proportions were respectively: 16.62%, 16.05%, 44.23%; And no-tillage and no-tillage+straw returning could significantly improve content of 5-2 mm water stable aggregates, the proportions were respectively: 29.81% and 64.28%, stabilities of soil aggregates were also improved to some extent; Implementing no-tillage could significantly improve organic carbon content of 5- 2 mm, 2-1 mm water stable aggregates; Except 2-1 mm under conventional tillage+straw returning treatment, straw returning improved organic carbon content of all the size water stable aggregates inordinately.