A one-dimensional simulation code for single cell intermediate-temperature disk-type seal-less SOFC is improved to include the air back diffusion phenomenon, which implies the diffusion of air into the fuel channel due to the mixing of the anode and cathode exhaust gases and the complete oxidation of hydrogen at the seal-less edge of the cell. The numerically obtained current density-single cell voltage (i-V) characteristic curves calculated by using the simulation code coincide well with the experimental ones. To investigate the effect of the air back diffusion phenomenon on the cell performance, the numerically obtained i -V curves without the air back diffusion phenomenon are compared with those considering the air back diffusion phenomenon. It is revealed that the single cell voltage reduction due to the air back diffusion phenomenon is negligible under normal operating conditions. However, it is made clear that the reduction of single cell voltage caused by the air back diffusion phenomenon becomes significant when the cell is operated under low fuel utilization conditions.