Abstract: The theoretical open circuit voltage (OCV) of solid oxide fuel cells (SOFCs) fed with carbon-based fuels and the amount of deposited carbon on the anode were calculated by the minimum Gibbs free energy method. The OCV values of Ni-YSZ Ⅱ YSZ LI LSM anode-supported cells using methane-based fuels were experimentally measured. The resuks indicate that the amount of deposited carbon decreases from the C-comer of the C-H-0 diagram to the carbon deposition boundary at a constant rate. In the case that most of deposited carbon is electrochemically oxidized, feeding fuels with high C:H ratio can enhance the value of OCV; Conversely, fuels with low C:H ratio are suggested. Theoretical analysis shows the rapidly decreasing value of OCV near the OCV boundary (OCV=0 V). Experimental results also reveal that the value of OCV dramatically decreases when the volume fraction of CO₂ in fuel gas increases to be higher than 80%. These results imply that the compositions of carbon-based fuels should be near the carbon deposition boundary in the C-H-0 diagram, which is beneficial to suppress carbon deposition without significant OCV reduction. A relatively high value of OCV can be obtained at the H-corner in the non-carbon-deposition zone near the carbon deposition boundary, and the sequence of the value of OCV is CH₄ > H₂ > CO in the CO₂-diluted ambiance at 600 ℃. Furthermore, the value of OCV cannot be significantly elevated via raising the proportion of external reforming or decreasing the temperature.