To determine the effect of the tumor necrosis factor-α (TNF-α) in odontoclast formation, we administrated a TNF-α inhibitor in rats with diabetes rats with periodontitis. The rats included in the study were divided into three groups: control rats without diabetes or periodontitis (the C group), rats with periodontitis and diabetes (the PD group), and rats with periodontitis and diabetes treated by infliximab, the TNF inhibitor (the PD+infliximab group). The PD and PD+ infliximab groups received intravenous administrations of streptozotocin (STZ, 50 mg/kg) to induce diabetes. After 7 days of STZ injections, the mandibular first molars were ligatured to induce periodontitis. The PD+infliximab group was intrapenitoneally administrated by infliximab (5 mg/kg). On days 3 and 20 after the ligature administration, odontoclast formation along root surfaces was evaluated by tartrate resistant acid phosphatase (TRAP) staining and cathepsin K immunohistochemistry. On day 3, the number of TRAP- and cathepsin K-positive cells increased more so in the PD group than in the C group. The PD+infliximab group showed a lower number of positive cells than the PD group. There was no difference in all the groups on day 20. On day 3, the cathepsin-K positive multinucleated and mononucleated cells were higher in the PD group than in the C group. The number of cathepsin-K positive multinucleated cells was lower in the PD+infliximab group than in the PD group. The PD group showed more cathepsin K-positive cells in the furcation and distal surfaces than the c group. The Cathepsin K-positive cells of the PD+infliximab group were lower than that of the PD group in furcation. These results suggest that TNF-α stimulates odontoclast formation in diabetes with periodontitis.