Nidus Vespae has been used in traditional Chinese medicine (TCM) to treat various cancers, but the underlying mechanisms were not yet clarified. This study was to investigate the effect of Nidus Vespae decoction (NVD) on tumor cell viability and immunoregulating functions of human peripheral blood immune cells. The effects on tumor cell viability, peripheral blood mononuclear cell (PBMC) proliferation activity, and the tumor cell phagocytosis of monocytes were evaluated by cell counting kit-8. Tumor-killing activity of cytotoxic T lymphocyte (CTL) was analyzed by (51)Cr releasing assay. IgG production of B cells and cytokine (TNF-α and IL-6) secretion of monocytes were determined by ELISA method. Data showed that NVD has no significant inhibiting effects on gastric cancer cells growth. Nevertheless, it could obviously promote PBMC proliferation in a time- and concentration-dependent manner. After treatment with NVD, the CTL cytotoxicity against SGC-7901 was significantly greater than control. The TNF-α and IL-6 secretion of monocytes and the IgG production of B cells also increased remarkably. Furthermore, NVD could significantly promote the phagocytosis of monocytes on tumor cells. These results suggest that NVD appears to have an immunoenhancing effect on immune cells, indicating that Nidus Vespae is worth exploring for immunomodulatory effects in tumor treatment.