〔Abstract〕 Objective To investigate the correlation between Yes-associated protein(YAP) nuclear expression and tumor size with prognosis of patients with epithelial ovarian cancer(EOC) and to study the role of YAP in EOC. Methods 120 patients with EOC were selected as the experimental group, including 38 patients with early stage(Ⅰ+Ⅱ) EOC and 82 patients with advanced stage(Ⅲ+Ⅳ) EOC. 30 normal ovarian tissues obtained from patients with uterine leiomyoma were enrolled as the control group. Immunohistochemical(IHC) assay was employed to determine YAP expression and sub-location. The relationship between YAP expression and the pathological parameters of the 120 patients with EOC was analyzed, so as to the prognosis of these patients. EOC cells(C13K and OV2008) were cultured with varying initial cell volumes. Ki67 expression and cell proliferation were tested by immunofluorescence and cloning assay respectively. YAP expression at mRNA and protein levels were detected by q-PCR and Western blot respectively when the cell conference of EOC cells reached to low(60%) and high(90%) cell density.Results The YAP nuclear expression was significantly higher in the EOC group compared to the control group(P<0.05). The average diameter of stage Ⅰ + Ⅱ EOC was larger than that of stage Ⅲ+ Ⅳ EOC(P<0.01). The high nuclear expression of YAP was positively associated with pathological grade,clinical stage and the level of Ca125>1 000 IU/ml, while negatively correlated with tumor size(allP<0.05).Survival analyses showed that smaller tumor size(<10 cm) and higher YAP nuclear expression were negatively associated with the 3-year overall survival rate of EOC patients(P<0.01). C13K and OV2008 cells cultured in the low density group exhibited a high number of clone formation, high Ki67 and YAP expression(P<0.01). The down-regulation of YAP expression could decrease the cell viability of EOC cells in the low-and high-density groups(P<0.05).Conclusion Higher level of YAP nuclear expression and smaller tumour size are inversely associated with the clinical prognosis of patients with EOC. Inhibiting YAP nuclear expression leads to a decrease in the proliferation capacity of EOC cells.