PF40 combined with metformin could significantly improve the symptoms of insulin resistance in type 2 diabetes mellitus (T2DM) rats, the molecular mechanism might be through inhibiting the expression of RORγ protein and increasing Foxp3 protein in the jejunum of T2DM rats, and then restoring the STZ-induced imbalance of T helper 17(Th17)/ regulatory T cells (Treg) cells, thereby maintaining intestinal immune homeostasis. This evidence concerns the gene FOXP3 and diabetes mellitus.