CDK8 is a cyclin-dependent kinase (CDK) member of the mediator complex that couples transcriptional regulators to the basal transcriptional machinery, and it has been investigated for possible tumor promoting functions. However, it is unclear whether CDK8 is involved in breast tumor cells growth. The aim of this study was to determine whether the suppression of CDK8 by small interfering RNA (siRNA) inhibits the growth of human breast cancer cell.CDK8-siRNA transfection was used to silencing the CDK8 gene in established breast cancer cell line, MDA-MB-231 and MCF-7, successful transfection being confirmed by Real-time PCR and could be shown by Western Blotting. CDK8 deletion caused significant decline in cell proliferation was observed in breast cancer cell lines as investigated by MTS assay, the number and size of the colonies formed were also significantly reduced in the absence of CDK8. Furthermore, transwell test were conducted to explore the migration of breast cancer cells. Moreover CDK8 gene knockdown arrested cell cycle.CDK8 mRNA expression was reduced after transfection with CDK8-siRNA, and protein expression had a similar trend. Transfection of CDK8-siRNA suppressed breast cancer cells proliferation and migration; meanwhile the cells were arrested at G0/G1 phase.CDK8 plays an essential role in breast cancer progression, which might inhibit the proliferation and migration in breast cancer cells.