Spatial and seasonal variabilities in methane emission from natural wetlands in the Sanjiang Plain, China were studied in the years 2001-2002 and the factors influencing methane emission were also evaluated. A seasonal variability of methane emission that increased with plant growth, reached maximum in August and then decreased was identified in freshwater marshes. This pattern was mainly affected by temperature and depth of standing water. Low methane emission at the early growth stage was due to low temperature rather than deficiency of substrate for methane production. Low temperature not only reduced directly methane production but also lowered the activity of aerobic microbes; the latter in turn left more oxygen in the rhizosphere for methanotrophs resulting in more methane to be oxidized. Methane emission in the freshwater marsh showed the following order: Carex lasiocarpa > Carex myeriana > Deyeuxia angustifolia. The Carex plants transported 72-86% of total methane emission, but only 28-31% for Deyeuxia angustifolia. Carex lasiocarpa accentuated methane oxidation rather than methane production in the rhizopshere and by contrast, Deyeuxia angustifolia stimulated methane production by excreting root exudates instead of methane oxidation by releasing oxygen. The higher methane concentration in porewater in the Carex lasiocarpa marsh than in Deyeuxia angustifolia marsh was mainly because of the deep standing water, which inundated more plant litters resulting in more organic carbon to be provided for methane production.