A free-air CO₂ enrichment (FACE) experiment was conducted during rice-growing season in 2005 in paddy fields at Jiangdu of Jiangsu province with two different N fertilization levels (Usual N-UN and Low N-LN) and two levels of rice-straw-returning-back-to-field (HR, NR) to evaluate population changes of methanogens and methanotrophs by the most-probable-number (MPN) method as well as methane production and oxidizing potential by gas chromatography. Results showed that elevated atmospheric CO₂ concentrations increased methanogens population at tillering stage, but restrained methanogens activity at heading and harvest stages under HR condition. Rising atmospheric CO₂ concentrations increased methanotrophs activity under LN condition, and methanotrophs population in FACE condition was significantly higher than that of Ambient in heading stage. But methanotrophs activities under UN and HR conditions were restrained except at heading stage. Methane production potential showed that FACE promoted methane release, especially under UN condition. High methane oxidizing activity was found under UN condition with enough exterior-CH₄ and rising atmospheric CO₂ concentrations.