Gold nanoparticles (AuNPs) have several potential biological applications as well as excellent biocompatibility. AuNPs with surface modification using polyethylene glycol (PEG-AuNPs) can facilitate easy conjugation with various biological molecules of interest. To examine the anticancer bioactivity of PEG-AuNPs, we investigated their effect on human chronic myeloid leukemia K562 cells. The results indicated that PEG-AuNPs markedly inhibited the viability and impaired the cell membrane integrity of K562 cells. The particles caused morphological changes typical of cell death, and a marked increase in the sub-G1 population in DNA histogram, indicating apoptosis. In addition, PEG-AuNPs reduced the mitochondrial transmembrane potential, a hallmark of the involvement of intrinsic apoptotic pathway in K562 cells. Observation of ultrastructure under a transmission electron microscope revealed that the internalized PEG-AuNPs were distributed into cytoplasmic vacuoles and damaged mitochondria, and subsequently accumulated in areas surrounding the nuclear membrane. In conclusion, PEG-AuNPs may have the potential to inhibit growth and induce apoptosis in human chronic myeloid leukemia cells.