p53 inactivation is a clinically defined characteristic for cancer treatment-nonresponsiveness. It is therefore highly desirable to develop anticancer agents by restoring p53 function.1 Herein the synthesized phthalazino[1,2-b]quinazolinones were discovered as p53 activators in bladder cancer cells. 10-Bromo-5-(2-dimethylamino-ethylamino)phthalazino[1,2-b]quinazolin-8-one (5da) was identified as the most promising candidate in view of both its anticancer activity and mechanisms of action. 5da exhibited strong anticancer activity on a broad range of cancer cell lines and significantly reduced tumor growth in xenograft models at doses as low as 6 mg/kg. Furthermore, 5da caused cell cycle arrest at S/G2 phase, induced apoptosis, changed cell size, and led to cell death by increasing the proportion of sub-G1 cells. Molecular mechanism studies suggested that accumulation of phospho-p53 in mitochondria after 5da treatment resulted in conformational activation of Bak, thereby evoking cell apoptosis, finally leading to irreversible cancer cell inhibition. Our present studies furnish new insights into the molecular interactions and anticancer mechanisms of phospho-p53-dependent quinazolinone compound.