Nickel nanoparticles (Ni NPs) have been applied in a wide range of areas because of their unique structure and properties such as catalysts, high-density magnetic recording media and others. However, little effort has been paid to their biological application and the concrete effect of Ni NPs on biological systems is still unknown. In this study, the possibility of the utilization of the magnetic Ni NPs in cancer cell studies was explored and the effects of the Ni NPs capped with positively charged tetraheptylammonium on leukemia K562 cells in vitro were investigated. Our observations of optical microscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies indicate that the morphological changes of cancer cells induced by Ni NPs could be apparently observed. The results of 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl-tetrazolium bromide (MTT) assay, DNA fragmentation and flow cytometry studies demonstrate that the Ni NPs could exert cytotoxicity to leukemia K562 cells at high concentration, and subsequently induce both apoptosis and necrosis of target cancer cells, whilst it had little impact on target cells when at low concentration. Meanwhile, functionalized Ni NPs with positively charged groups could enhance the permeability of cell membrane and facilitate the cellular uptake of outer target molecules into cancer cells. These findings reveal the potential mechanism of Ni NPs to target cancer cells which could induce the cytotoxicity to leukemia cancer cells and suggest the possibility for applications of the Ni NPs in related clinical and biomedical areas.