Abstract: In this study the cytotoxicity caused by HMSN to human umbilical vessel endothelial cells (HUVECs) was examined by inhibition of cell proliferation, lactate dehydrogenase (LDH) release assay, flow cytometer analysis, transmission electron microscopy (TEM) and indictive coupled plasma-atomic emission spectroscopy (ICP-AES). The results showed that HMSN at the concentration more than 62.5 μg/mL will exert cytotoxicity on HUVECs and the IC50 is 150 μg/mL. At high concentration HMSN significantly inhibited the proliferation of HUVECs, causing LDH release, arrest cell cycle at G1 period and eventually induced necrosis. The extracts of HMSN also had the same effect on HUVECs at different dilution ratios less than 1:64, indicating the degradation product of HMSN had cytotoxicity. The cytotoicity may come from the physical damage on cell structure by HMSN, and chemical disturbance on metabolism caused by product of HMSN degradation. Therefore, attentions should be paid on the single dosage, avoiding cytotoxicity to normal tissue vessels; stimulating cytotoxicity to tumor vessel.