To evaluate the effect of cultivation, nitrogen fertilizer, and set aside on CH4 uptake after drained marshland was converted into agricultural fields, CH4 fluxes and CH4 concentrations in soil gas were in situ measured in a drained marsh soil, a set-aside cultivated soil, and cultivated soils in Sanjiang Plain of Northeast China in August 2001. Over the measuring period, the highest CH4 uptake rate was 120.7±6.2 μg CH4 m−2 h−1 in the drained marsh soil and the lowest was 29.5±4.9 μg CH4 m−2 h−1 in the set-aside cultivated soil, showing that there was no significant recovery of CH4 uptake ability 5 years after cultivation activity was stopped. CH4 uptake rates were significantly less in the cultivated soils than in the drained marsh soil by 30.1–74.6%, which resulted mainly from cultivation and partly from nitrogen addition. A significantly negative correlation between CH4 flux and bulk density in the cultivated soils tilled by machine suggests that cultivation reduced CH4 uptake through compaction, because of the enhanced diffusion resistance for CH4 and O2. Nitrogen fertilization slowly reduced but persistently affected CH4 uptake even after long-term application of nitrogen.