α-Galactosylceramide (α-GalCer) has been reported to be therapeutic against metastatic liver tumors in mice. However, little is known regarding the efficacy of combined chemo-immunotherapy using α-GalCer and anticancer drugs. In this study, we evaluated the antitumor effect of the combination therapy of α-GalCer and 5-fluorouracil (5-FU) against liver tumors of MC38 colon cancer cells. The liver weights of tumor-bearing mice treated with the combination were significantly lower than those of nontreated mice and of mice treated with 5-FU or α-GalCer alone. No toxic effects on the liver and renal functions were observed in any of the treatment groups. α-GalCer treatment induced significant activation of liver NK cells in vivo, but 5-FU treatment did not. 5-FU treatment resulted in a significant upregulation of NKG2D activating molecules (Rae-1 and H60) and DNAM-1 ligands (CD112 and CD155) on MC38 cells, but α-GalCer did not. The cytolytic activity of α-GalCer-activated liver mononuclear cells against 5-FU-treated MC38 cells was significantly higher than that against nontreated cells. The increase of the cytolytic activity induced by 5-FU partially depended on NKG2D-Rae-1 or H60 signals. Depletion of NK cells significantly inhibited the antitumor efficacy of 5-FU against MC38 liver tumors, which suggested that the antitumor effect of 5-FU partially depended on the cytolytic activity of NK cells. These results demonstrated that the combination therapy of α-GalCer and 5-FU produced synergistic antitumor effects against liver tumors by increasing the expression of NK activating molecules on cancer cells. This study suggests a promising new chemo-immunotherapy against metastatic liver cancer.