The tumor suppressor p53-mediated apoptotic response plays an important role in cisplatin resistant in ovarian cancer. The adenovirus (Ad) type 12 E1B 55-kDa protein binds to p53 and inactivates its transcriptional transactivation function. In this study, we test the hypothesis that Ad12 E1B 55-kDa oncoprotein promotes p53-mediated apoptotic response of ovarian cancer to cisplatin. First, we observed the upregulation protein level of p53 target genes in cisplatin-resistant or cisplatin-sensitive ovarian cancer by Western blotting. Second, after transfection of Ad12 E1b 55-kDa expression plasmid, the expressions of p53 target genes in A2780 cells were further enhanced. Co-IP experiment demonstrated Ad12 E1b 55 kDa associated with p53. MTT assay confirmed that the cell proliferation was enhanced after transfection, as well as the enhanced cell inhibitory rate in the presence of cisplatin. Using flow cytometry, transfection of Ad12 E1B 55-kDa protein induced apoptosis and promoted S-phase transition in proliferation. Finally, results showed that all these changes promoted by Ad12 E1b 55 kDa were attenuated by the exposure of specific inhibitor of p53 signaling, pifithrin-α. Taken together, we concluded that Ad E1B 55-kDa oncoprotein promotes p53-mediated apoptotic response of ovarian cancer to cisplatin.