To understand the summertime variation pattern of CH4 oxidation in the rhizosphere, CH4 oxidation was evaluated in a freshwater marsh vegetated with Carex lasiocarpa and Carex meyeriana in the Sanjiang plain of northeast China at the three growing stages. Two methods through covering plants with a black cloth and applying acetylene to inhibit CH4 oxidation were used to quantify CH4 oxidation rates in the field. CH4 fluxes and CH4, dissolved organic carbon (DOC) and CO2 concentrations in porewater in the freshwater marsh were determined before and after the plants were covered or acetylene was applied. Temperature and redox potential in the vertical profile as well as aboveground and underground plant biomass were measured as well. Due to CH4 oxidation, CH4 concentrations in porewater in the C. lasiocarpa and C. meyeriana marsh were reduced by 2.1–18.2% and 0.7–25.8%, and the fluxes of potential CH4 emissions were lowered by 3.2–35.9% and 4.3–38.5%, respectively. The highest CH4 oxidation rate occurred in June, up to 30.1–38.5% of the potential CH4 fluxes, whereas the correspondent value was only 3.2–15.8% in July and August. CH4 oxidation rates in the Carex marsh decreased with increasing temperature in the vertical profile over the growing season. This is due likely to more O2 released from the roots in porewater to be consumed by other aerobic microbes in the rhizosphere at high temperature, resulting in the decrease of the CH4 oxidation rate.