Hepatocellular carcinoma (HCC) is the most common liver malignancy with dismal prognosis and limited treatment options. Natural killer (NK) cells are critical components of antitumor immunity due to their capacity to eliminate MHC class I-deficient cells. To evaluate the function of NK cells in HCC patients, circulating CD3(-)CD56(+) NK cells were collected from HBV-associated HCC patients and healthy control individuals. Compared to NK cells from healthy controls, NK cells from HCC patients presented functional impairment, characterized by significantly reduced cytotoxicity, degranulation, and cytokine production. Exogenous IL-21 could reinvigorate NK cells from HCC patients, resulting in significantly increased levels of cytotoxicity, de granulation, and cytokine expression. However, IL-21-treated NK cells from HCC patients still presented lower response than IL-21-treated NK cells from healthy controls. IL-21 resulted in increased phosphorylation of both STAT1 and STAT3 in NK cells. Inhibition of STAT1, but not STAT3, significantly reduced IL-21-mediated reinvigoration of NK function. Together, this study demonstrated that NK cells in HBV-associated HCC patients presented functional impairments that could be reverted by IL-21 in a STAT1-mediated mechanism.