New strategies that stimulate cell-mediated immunity (CMI) against tumors and inhibit regulatory T cells are needed to improve the outcome of cancer immunotherapy. The aim of this study was to enhance the anti-tumor immunity of gp96 vaccine through naloxone administration. Therefore, we used BALB/c mouse model of fibrosarcoma tumor and analyzed the tumor size, splenocyte proliferation, spleen and tumor-infiltrated lymphocytes. Tumor and spleen CD4+CD25+Foxp3+ regulatory T lymphocytes, cytotoxic activity of the splenocytes, IFN-gamma and IL-4 secretion were assessed to describe the anti-tumor immune response. Our findings showed that co-administration of gp96 and naloxone has resulted in a significant reduction in CD4+CD25+Foxp3+ regulatory T cells in the spleen. The results indicated that on days 27 and 32 the tumors in the gp96+Nal group grew significantly slower. Moreover, co-administration of gp96 and naloxone significantly increased the intra-tumor CD8+ T cells and cytotoxic activity. In addition the results indicated a significant increase in the proliferation of splenocytes and IFN-gamma production. Our results demonstrate that naloxone is an effective immunoadjuvant in cancer immunotherapy.