Solid oxide fuel cells are studied under direct methane feeding with 10-70% CH4. When either La0.58Sr0.4Co0.2Fe0.8O3-delta (LSCF)-Ce0.9Gd0.1O1.95 (GDC) or Ni-added LSCF-GDC composite is used as the anode, the oscillations of the electrical current and the formation rates of CO and CO2 occur. The oscillation of the electrical current can be explained by a mechanism of periodic oxidation-reduction of the bulk lattice of the anode, with the determining factor being the build-up of the concentration of the oxygen vacancies to a certain extent. As the methane concentration increases, the current density increases and becomes larger with Ni addition. Higher methane concentration leads to higher possibility to induce the oscillation, to start it earlier, and to result in a larger amplitude. Ni addition inhibites the occurrence of the oscillation of the electrical current but promotes that of the CO2 formation rate. (C) 2009 Elsevier B.V. All rights reserved.