Natural killer T (NKT) cell activation is responsible for eliminating pathogens. However, the biological functions of NKT cells against influenza virus are not fully understood. We therefore investigated the effects of NKT cells in viral infection using CD1d knockout (KO) mice. When CD1d KO or wild-type (WT) mice were infected with a sub-lethal dosage of the influenza virus, the survival rate of CD1d KO mice was significantly lower than for WT mice in association with delayed viral clearance in the lungs. Consistently, IFN-γ production in bronchoalveolar lavage fluid of CD1d KO mice was largely reduced compared to WT mice during infection. Moreover, the cytotoxic activities of NK cells and viral antigen-specific CD8⁺ T cells were impaired in CD1d KO mice. It was concluded that activated NKT cell-induced IFN-γ release enhances both NK-cell activity and antigen-specific CD8⁺ T cells to eliminate the influenza virus, thus leading to an enhanced survival.