Abstract: The aim of this study was to investigate the effect of transmembrane 9 superfamily protein member 2 (TM9SF2) in proliferation and migration of triple negative breast cancer cell line MDA-MB-231.The expression of TM9SF2 in triple negative breast cancer cell line MDA-MB-231 and nontumorigenic mammary epithelial cell line MCF-10A were measured by Western blot. MDA-MB-231 cells were treated with siRNA-TM9SF2 to knock-down the expression of TM9SF2. The effect of silencing TM9SF2 was measured with Western blot.The proliferation of cells was tested by MTS,and the migration was measured with Transwell and wound-healing assay.Proteins related to proliferation (PI3K,AKT,SRC and ERK) and migration (Snail,Slug and N-cadherin) were measured with Western blot.Protein expressions of TM9SF2 was better improved in triple negative breast cancer MDA-MB-231 cell line than MCF-10A.Compared with the control group, the siRNA-TM9SF2 infected group had lower expressions of PI3K, Snail, Slug and N-cadherin, and at the same time phosphorylation of AKT was decreased. The results suggest TM9SF2 can promote the proliferation and metastasis of triple negative breast cancer MDA-MB-231 cell line.