This study examines the performance of a metal-based solid oxide fuel cell (SOFC) coupled with an integrated diesel fuel processor, with a focus on operating stability. The reformate is produced by an autothermal reformer (ATR), desulfurizer, and post-reformer using commercial diesel. Diesel reformate possesses the characteristics of low fuel concentration and high steam quantity due to its fuel processing condition for stable operation. These characteristics lead to high oxygen partial pressure at the anode which causes the oxidation of the cell's metallic component. Various approaches, such as thermodynamic calculation, temperature-programmed reduction (TPR) analysis, and the electrochemical performance of a single cell, are used to prove the oxidation phenomenon in this experiment. As a result, the key factors for stable operation are discovered, and a metal-based SOFC single stack with an area of 50-mm x 50-mm is successfully operated for 1000 h at a 4%/1000 h degradation rate under a modified condition of diesel reformate. To ensure the SOFC's stable operation using the reformate, it is recommended that the operating condition should be considered from a viewpoint combining fuel composition, oxygen partial pressure, and temperature. The modified condition for the long-term operation of a metal-based SOFC using diesel reformate is also suggested in this paper.