Synergistic antitumor effects of HB (berberine α-hydroxy-β-decanoylethyl sulfonate, houttuyn berberine) with HCPT (hydroxycamptothecine), and its correlative mechanism were studied in vitro.  MTT assay was employed to determine the cytotoxicity of HB combined with HCPT in tumor cells culture in vitro, IC₅₀ and combination index (CI value) were used to evaluate the synergistic effects.  The supercoiled DNA relaxation mediated by topoisomeraseⅠ&Ⅱ was measured by agarose gel electrophoresis assay, and influence of HB was detected.  The results showed that HB could inhibit the proliferation of tumor cells (SGC-7901, SW1116 and SW480) in vitro, and the inhibition ratio was increased, IC₅₀ was reduced when combining with HCPT.  CI value of the two drugs was less than 1 in HepG2, SW480, SGC-7901 and SW1116 cells.  The lowest value was 0.447, 0.626, 0.161 and 0.178 in these tumor cells, respectively, further indicating HB has synergistic action with HCPT on suppressing tumor proliferation.  The agarose gel electrophoresis assay showed HB can inhibit topoisomeraseⅠ& Ⅱ activity of SW480 cells at the concentration of 2.0−8.0 mg·L⁻¹.  HCPT is a typical inhibitor of topoisomeraseⅠ, the synergistic action between HCPT and HB on suppressing tumor proliferation is perhaps related to the congenerous inhibition of topoisomerase.