Abstract: Bromodomain-containing proteins (BCPs) can specifically recognize acetylated lysine (KAc) in histones and other substrate proteins. Recently, several kinase inhibitors were found to inhibit bromodomains, such as the PLK1 inhibitor BI-2536 and the JAK2 inhibitor TG101209, which bind to BRD4 with IC₅₀ values of 25 nmol·L^-1 and 130 nmol·L^-1, respectively. To obtain potent BRD4 inhibitors from inhibitor BI-2536, we used dihydroquinoxalin-2(1H)-one to replace the 7,8-dihydropteridin-6(5H)-one in BI2536. By exploring the structure-activity relationships of the new dihydroquinoxalin-2(1H)-one structures, we obtained a novel phenyl side chain series of BRD4 inhibitors. We identified several potent BRD4 inhibitors, especially compounds 16, 22, 28 and 29, which had IC₅₀ values below 100 nmol·L^-1 in fluorescence anisotropy (FA) assays, indicating this series of compounds are worth to fruther investigation.