Overexpression of human ether-a-go-go (eag) related gene (hERG) has been found in a broad range of human leukemia cell lines and primary human leukemia. The block of hERG protein might he a potential therapeutic strategy for leukemia. Gambogic acid (GA) has recently exhibited marked anti-tumor potency on solid tumors of various derivations. Here, we investigated the anti-leukemia effects of GA and its relation to the regulation of hERG in K562 leukemia cells in vitro.K562 cells were treated with various concentrations of GA (0.125-8.0 μmol/L) for 0-72 h. MTT assay was used to evaluate the inhibition effect of GA on the growth of K562 cells. Cell apoptosis was meas-ured through both Annexin-V FITC/PI double-labeled cytometry and transmission electron microscopy.Cell cycle regulation was studied by a propidium iodide method. RT-PCR and West blot were applied to detect the expression level of hERG in K562 cells. GA presented striking growth inhibition and apoptosis induction potency on K562 cells in vitro in a time- and dose-dependent manner. The IC50 value of GA for 24 h was 2.637±0.208 μmol/L. Moreover, GA induced K562 cells arrested in G0/G1 phase, accordingly, cells in S phase decreased gradually, and no obvious changes were found in G2/M phase cells. Under the transmission electron microscopy, apoptotic bodies containing nuclear fragments were found in GA-treated K562 cells. After treatment with GA of 2.0μmol/L for 24 h, the percentage of apoptotic cells was increased from 4.09% to 18.47% (P<0.01), Overexpression of hERG channel was found in K562 cells, while GA could down-regulate it at both protein and mRNA levels (P<0.01). It was concluded that GA exhibited its anti-leukemia effects partially through down-regulating the expression level of hERG channel in K562 cells, suggesting that GA may be a potential agent against leukemia with a mechanism of blocking hERG channel.