Aim : Natural killer (NK) cells are cytotoxic lymphocytes that lyse certain tumor- and virus-infected cells without any prior stimulation or immunization. This article aims to review the significance of evaluating peripheral blood NK cells to predict anti-tumor immune function and prognosis in cancer patients. Methods : PubMed was used to create a database for this review. Search words of cancer, natural killer cell, prognosis were used to retrieve related articles. References of the collected articles were also reviewed. Results : Current evidence indicates that decreased or absent NK cell count or activity is mostly associated with the development or progression of cancer. In patients with various types of cancer, NK cell activity was mostly associated with the cancer prognosis and survival despite some conflicting results. Conclusion : The data shows the evaluation of anti-tumor immunity in cancer patients through natural killer cell measurement still remains a controversial matter. However, it is clear that the NK cell activity plays an important role in cancer and is associated with prevention of both early stage and metastatic cancer.