Abstract: Disclosed is a method of culturing natural killer cells (NK cells) applied to immunotherapy. More specifically, disclosed are a kit containing a medium for culturing NK cells (NKCM kit) that can efficiently amplify and activate NK cells effective for the treatment of malignant tumors by culturing lymphocytes derived from human peripheral blood, and a method of culturing natural killer cells using the kit. The method for amplifying NK cells of the present invention includes stimulating NK cells with lymphocytes separated from peripheral blood, culturing the NK cells in a medium containing IL-2, IL-12, IL-15, IL-17, IL-18, and IL-21, and isolating the NK cells. Provided is a pharmaceutical composition for cell therapy containing NK cells produced by the method of amplifying NK cells. The pharmaceutical composition for cell therapy is expected to be widely used to treat infections and/or cancer.