Natural killer (NK) cells exhibit cytotoxicity against variety of tumor cells and virus-infected cells without prior sensitization and represent unique lymphocytes involved in primary host defense. NKR-P1 is thought to be one of NK receptors mediating activation signals because cross-linking of NKR-P1 activates NK cells to exhibit cytotoxicity and IFN-γ production. However, molecular mechanism of NK cell activation via NKR-P1 is not well elucidated. In this study, we analyzed the cell surface complex associated with NKR-P1 on NK cells and found that NKR-P1 associates with the FcRγ chain which is an essential component of Fc receptors for IgG and IgE. The association between FcRγ and NKR-P1 is independent of Fc receptor complexes. Furthermore, NK cells from FcRγ-deficient mice did not show cytotoxicity or IFN-γ production upon NKR-P1 cross-linking. Similarly, NK1.1 + T cells from FcRγ-deficient mice did not produce IFN-γ upon NKR-P1 crosslinking. These findings demonstrate that the FcRγ chain plays an important role in activation of NK cells via the NKR-P1 molecule.