Objective: Olaparib, one of the first PARP inhibitors, has been used for maintenance therapy in BRCA-mutant patients with platinum-sensitive, recurrent, high-grade serous ovarian cancer. In acute myeloid leukemia(AML) and breast cancer cells, DNA-demethylating agents, inhibitors of DNA methylation and DNA methyltransferases(DNMTs), increase PARP1 tightly bound into chromatin. The purpose of this study is to confirm anti-cancer effect of combination therapy, Olaparib with DNA Demethylating agents(5-Azacytidine), in ovarian cancer cell models. Methods: In vitro experiments, we treated Olaparib only, 5-Azacytidine only, and combination with Olaparib and 5-Azacytidine in ovarian cancer cell lines(A2780, SKOV3ip1, HeyA8) to evaluate the effect on cell proliferation using MTT assay. To check the human Active caspase3, MMP2, and MMP9 level and apoptosis according to each treatment, we performed Western blot, ELISA, and apoptosis assay. With wound-healing assay and migration assay, invasive ability of cancer cell after treatment of each inhibitor was compared. For in vivo experiments, cell line xenografts with A2780 and PDX models with implanting tumor(<2-3mm) into subrenal capsule of kidney were used. We recorded tumor weight and number of tumor. Results: Both Olaparib and 5-Azacytidine inhibited the cell survival and increased the apoptosis in ovarian cancer cells. In this study, combination treatment of cells with Olaparib and 5-Azacytidine significantly inhibited the cell growth and increased the apoptosis compared to the single agent treatment in ovarian cancer cells. In in vivo experiments, combination treatment with Olaparib and 5-Azacytidine significantly decreased weight and nodule numbers of tumor in cell-line xenograft models with A2780 cells and PDX model compared with other groups. Conclusion: We found that combination treatment with Olaparib and 5-Azacytidine has synergetic effect to attack ovarian cancer cells compared with control or single agent treatment through in vitro and in vivo tests.