Anodic reaction mechanisms of a direct methanol fuel cell (DMFC) with vapor feed were studied. Membrane electrode assemblies with different catalysis loads were fabricated and AC impedance spectroscopy was measured. The spectrum was deconvolved into three elementary processes, which showed different polarization dependences. The medium-frequency process was considered to be the process related to methanol electrode oxidation and was dominant at a low polarization condition. The low-frequency process was observed only when the methanol concentration was low. This indicates that the removal of reaction product gas does not hinder the anodic reaction of the DMFC with vapor feed.