Natural Killer (NK) cells are a type of blood immune cells that are capable of lyse the infected or transformed cells without prior sensitization. Due to a small fraction of NK cells within human bodies, and the fact that the cells are not able to expand to lyse the target cells, cells were isolated from human blood, and were expanded. NK cells expanded without feeder cells showed only a small number of fold expansion. However, when the NK cells were co-cultivated with artificial Antigen Presenting Cells (aAPCs), NK cells showed a much greater number of fold expansion. Especially, NK cells co-cultured with K562 cells with membrane bound IL-21 (K562.mbIL21) showed about 10,000-fold expansion for 14 days cultivation. Continuous cultivation of NK cells with the K562.mbIL21 showed more than 100 billion-fold expansion for 30 days before it shows senescence. By employing a multivariate analysis technique, phenotypic changes during the activation and expansion of NK cells were captured. In addition, various kinetic parameters during the cultivation were identified to provide preliminary data for future research. It was found that NK cells could expand with dead aAPCs and their debris, showing a possibility of expanding NK cells without feeder cells since the K562 cells are cancerous, thus, they need to be completely removed for clinical use in promoting NK cells in the field of adoptive immunotherapy.