Objective:To explore the influence of Janus kinase inhibitor AG490 on phosphorylation of extracellular signal-regulated protein kinase (ERK) in the human breast cancer cell line MDA-MB-231,so as to discuss the effect of JAK on ERK signal transduction pathway and on invasion/metastasis of breast cancer cells. Methods: MDA-MB-231 cells were treated with Janus kinase inhibitor AG490; MTT assay was used to examine the proliferation of MDA-MB-231 cells and the adhesion of MDA-MB-231 cells to artificial basement membrane matrigel after AG490 treatment. Invasion and metastasis of MDA-MB-231 cells were evaluated with transwell chamber after treated with AG490.The expression of P-ERK and MMP-9 protein was determined by Western blotting assay. Results: AG490 inhibited the proliferation of MDA-MB-231 cells in a time- and dose-dependent manner; AG490 also depressed the adhesion, invasion and metastasis of MDA-MB-231 cells. Meanwhile, the expression of P-ERK and MMP-9 protein was decreased after treatment with AG490. Conclusion: Our study indicates that JAK kinase can affect the activity of ERK signal transduction pathway through the phosphorylation of ERK. The inhibitory effects of JAK kinase on MMP-9 expression and invasion of breast cancer cells are associated with the blocking of the ERK signaling pathway.