The effect of selenic acid cartilage polysaccharide (CA-Se) on the mitochondrial membrane potential of K562 human chronic myeloid leukemia cells and on the induction of apoptosis of K562 cells were studied. The inhibitory effect of CA-Se on the proliferation of K562 cells was studied using the MTT (3,-(4,5-dimethylthiazol-2-y))-2,5-diphenyl tetrazolium bromide) colorimetric assay, while the changes in the morphological characteristics after Hoechst 33342/PI staining were observed under a microscope. The changes in the mitochondrial membrane potential (ΔΨm) after rhodamine-123 staining were assessed by flow cytometry (FCM), and the protein and gene expressions of caspase-9 were analyzed by immunofluorescence and real-time polymerase chain reaction (RT-PCR), respectively. The results showed that the proliferation of K562 cells was significantly inhibited by CA-Se in a dose- and time-dependent manner. After CA-Se treatment, the morphological characteristics of cells were dramatically changed, and a reduction in cell volume, irregularities in cell membranes, and chromatin condensation were observed. The ΔΨm values reduced after CA-Se treatment from 96.10% (the control) to 78.07% (CA-Se treatment). The protein and gene expressions of caspase-9 both increased significantly. All the results indicated that CA-Se can induce apoptosis of K562 cells by reducing the ΔΨm, upregulating the expression of the caspase-9 (initiator caspase) gene, and activating caspase-9 protein.