Abstract: The cyclin-dependent protein kinase 9(CDK9)is a member of the family of cyclin-dependent protein kinases. Different from other CDKs, CDK9 mainly works on the transcription regulation and has no affects on the cell cycle progress. There are many kinds of CDK9 inhibitors in the clinical research. The detailed structure and action mechanism of CDK9, its difference of protein structure from other CDKs, several selective or nonselective CDK9 inhibitors, as well as their structure-activity relationship(SAR)are discussed in this paper.