Objective To evaluate effects of cucurbitacin Ⅰ on in vitro proliferation of HaCaT cells and the expression of keratin 17 (K17),signal transducer and activator of transcription 3 (STAT3) and vascular endothelial growth factor (VEGF) in cultured HaCaT cells.Methods In vitro cultured HaCaT cells were divided into 6 groups to be treated with cucurbitacin Ⅰ at different concentrations of 0.0125,0.025,0.05 and 0.1 μmol/L (0.0125,0.025,0.05 and 0.1 μmol/L cucurbitacin Ⅰ groups),DMEM containing the same volume of DMSO as 0.1 pmol/L cucurbitacin Ⅰ (DMSO group),DMEM (negative control group) and 10 nmol/L calcipotriol (positive control group),respectively.Cell counting kit-8 (CCK8) assay was performed to assess in vitro cellular proliferative activity after 12-,24-,36-hour treatment,reverse transcription (RT)-PCR to measure the mRNA expression of K17 and VEGF in HaCaT cells after 24-hour treatment,and Western blot analysis to determine the protein expression of K17,STAT3,phosphorylated-STAT3 (p-STAT3) and VEGF after 24-hour treatment.Statistical analysis was carried out by one-way analysis of variance (ANOVA),repeated measures ANOVA,Student-Newman-Keuls (SNK)-q test and Pearson correlation analysis.Results The proliferative activity of HaCaT cells started to decrease after 12-hour treatment with cucurbitacin Ⅰ at the concentration of 0.0125 μmol/L.When the concentration of cucurbitacin Ⅰ increased up to 0.1 μmol/L,the cell proliferation rates were inhibited by 43.00％ ± 2.11％ and 48.98％ ± 2.27％ after 24-and 36-hour treatment respectively.Compared with the negative control group,cucurbitacin Ⅰ at different concentrations all could inhibit in vitro proliferation of HaCaT cells (all P ＜ 0.05),and the inhibitory effects increased gradually with the increase of cucurbitacin Ⅰ concentration and treatment duration.After 24-hour treatment,the mRNA expression of K17 and VEGF and the protein expression of K17,VEGF and P-STAT3 were significantly decreased along with the increase of concentration of cucurbitacin Ⅰ (all P ＜ 0.05).Conclusion Cucurbitacin Ⅰ can inhibit in vitro proliferation of HaCaT cells,and down-regulate the mRNA expression of K17 and VEGF and protein expression of K17,VEGF and P-STAT3.