Radiotherapy (RT) is a potent anti-tumor modality. However, unwanted effects including increased recurrence and metastasis that involve factors such as cytokines, which induce complex molecular mechanisms, have also been reported. In a previous study, we showed that interleukin (IL)-12 and radiotherapy combination treatment suppressed tumor growth and metastasis in a hepatoma mouse model. In this study, we investigated the mechanism underlying the IL-12 anti-tumor effect during radiotherapy. In tumor-bearing mice, irradiation decreased IL-12 expression in the tumors and spleens. However, a number of dendritic cells infiltrated into the tumors in which IL-12 expression did not decrease. To further study the underlying detailed mechanism for this decrease in IL-12, LPS-stimulated bone marrow-derived dendritic cells (BMDCs) were irradiated, and then IL-12- and IL-6-associated molecules were examined in irradiated tumors and BMDCs. Irradiation resulted in IL-12 suppression and IL-6 increase. IL-6 and signal transducer and activator of transcription 3 (STAT3) inhibitors restored the irradiation-induced IL-12 decrease via suppression of C-Rel activation. Taken together, our study suggests that irradiation-induced IL-6 can decrease IL-12 production through the inhibition of C-Rel phosphorylation by the IL-6/STAT3 signaling pathway.