Immunotherapy has emerged as a promising therapeutic approach for treating several forms of cancer. Adoptive cell transfer of immune cells, such as natural killer (NK) cells, provides a powerful therapeutic potential against tumor cells. In the past decades, two-dimensional (2D) tumor models have been used to investigate the effectiveness of immune cell killing. However, the 2D tumor models exhibit less structural complexity and cannot recapitulate the physiological condition of the tumor microenvironment. Thus, the effectiveness of immune cells against tumor cells using these models cannot fully be translated to clinical studies. In order to gain a deeper insight into immune cell-tumor interaction, more physiologically relevant in vivo-like three-dimensional (3D) tumor models have been developed. These 3D tumor models can mimic the dynamic cellular activities, making them a much closer representation of the in vivo tumor profiles. Here, we describe a simple and effective protocol to study the cytotoxic activity of primary human NK cells toward the 3D tumor spheroids. Our protocol includes isolation and expansion of human NK cells, labeling and formation of tumor spheroids, co-culture of NK cells and tumor spheroids, and evaluation of cytotoxic activity using a confocal microscope. This protocol is also applicable to other types of tumors and immune cells.