Background Tumor cells can reduce the number of dendritic cells （DCs） in the tumor environment and cause DC dysfunction through autocrine or paracdne pathways. We sought to measure cyclooxygenase-2 （COX-2） expression in bombesin-inhibited DCs treated with theanine in vitro and to explore the protection and activation effects of theanine on DCs. Methods Enzyme-linked immunosorbent assay （ELISA）, reverse transcription-polymerase chain reaction （RT-PCR）, and Western blotting were used to analyze the effects of theanine on COX-2 expression and interleukin （IL）-12/IL-10 secretion of bombesin-treated DCs. Results DCs acquired an impaired phenotype as a result of bombesin treatment. Theanine increased the expression of mature DC surface molecules. The number of cell apoptosis with the treatment of bombesin and theanine significantly decreased, accounting for 15.9%, compared with 26.1% of cell apoptosis with bombesin. COX-2 expression in bombesin- treated DCs was inhibited by theanine in a dose-dependent manner. Theanine promoted DC secretion of IL-12. IL-12 levels reached （137.4_＋4.9） pg/ml with theanine at 200 pmol/L. However, theanine inhibited the secretion of IL-10 in a dose-dependent manner. IL-10 levels were only （58.4_＋6.9） pg/ml with theanine at 200 IJmol/L. Conclusion Theanine inhibits the transcription and translation of COX-2 and regulates the balance of IL-10/IL-12 secretion in bombesin-inhibited DCs, leadincl to the recovery of a state of activation in DCs.