Durability of the segmented cell-stacks was investigated. A cell stack is composed of multiple single-cells and interconnectors placed on both sides of a flat-tube substrate. Durability tests in an electric furnace were conducted under a standard condition as well as under controlled severe conditions in which a particular test parameter was set to severe value in order to define accelerating factors of deterioration. In the real system environment, the degree of variation of the parameters is larger than that in the standard condition in electric furnace. Therefore in order to estimate the degradation phenomenon in the real system, the durability tests were conducted also in the simulated system condition in furnaces with temperature distribution of 120 °C and reformed methane as a fuel. Cell-stacks under the "standard" condition at a current density of 0.24A/cm2, fuel utilization of 70%, and temperature of 775 °C with small temperature distribution less than 30 °C showed stable performance for experimental period up to 4000hrs. The degradation rates of the cell-stacks were found to be 0.27-1.04%/1000h with an average degradation rate of 0.6 %/1000h. Two accelerating factors of deterioration have been found through durability tests under controlled severe conditions. Then cell-stacks were modified successfully to improve the durability.