Prostaglandin (PGE2), synthesized by cyclooxygenase-2 (COX-2), is associated with cellular immune tolerance during the process of cancer development. Induction of tolerance requires a specific environment in which dendritic cells and regulatory T cells (Tregs) play an essential role. It was recently shown that maturation of dendritic cells in the presence of indoleamine 2, 3-dioxygenase (IDO) results in activation of Tregs, and inhibition of COX-2 activity regulated IDO expression within the tumor microenvironment. Thus, we hypothesized that the tumor immune tolerance would be inhibited by COX-2 inhibitor and this inhibition would be mediated by IDO-dependent Tregs inhibition. The PGE2 in Lewis lung cancer cells (3LL) and serum of mice were measured for the evaluation of COX-2 inhibitors' local and systemic effects. The production of PGE2 in 3LL cells and serum of 3LL tumor-bearing mice were decreased by COX-2 inhibition. However, there were no significant differences in serum PGE2 levels among normal control and celecoxib-treated nontumor-bearing mice. The accumulation of Tregs was reduced in the celecoxib-treated 3LL tumor-bearing mice. In addition, the expressions of COX-2, IDO, and Foxp3 were reduced in the mice treated with a COX-2 inhibitor, and this was found to correlate with a reduction in the size of tumor mass and metastasis. These results suggest that the antitumor effects of COX-2 inhibitors seemed to be correlated with the inhibition of IDO and Tregs. Therefore, COX-2 inhibitors might provide a therapeutic strategy for Tregs-induced tumor immune tolerance.