The activity loss of anode PtRu/C catalyst in direct methanol fuel cell (DMFC) was examined by an electrochemical aging test. The results show that after the aging test, the activity of PtRu/C catalyst toward methanol oxidation decreased obviously. The transmission electron microscopy and X-ray diffraction analysis display that catalyst particles experienced Otswald ripening and directional migration under the electric field during the aging test. The electrochemical impedance spectra show that after the aging test, both the ohmic resistance and the polarization impedance increased remarkably, which is related to the loss of active components. The energy dispersive X-ray analysis shows that the mass ratio of Pt to Ru in the catalyst layer increased noticeably after the test. It is suggested that the loss of active components due to the potential fluctuation is an important factor responsible for the activity loss of the anode catalyst in DMFC.