The transmembrane and secreted protein delta-like 1 homolog (DLK1) belongs to the EGF-like family. It is widely accepted that DLK1 plays important roles in regulating cell differentiation, such as adipogenesis and osteogenesis. Aberrant expression of DLK1 has been found in various types of human cancers, including lung cancer. A previous study in this lab has revealed that DLK1 is associated with tumor invasion, although the mechanism is still unknown. To explore the potential effects that DLK1 might have on invasion, DLK1 was overexpressed or knocked down in the human lung cancer cell lines. The protein's influences on cell invasion were subsequently evaluated. A transwell assay showed that DLK1 overexpression significantly promoted cancer cell invasion. Western blotting and gelatin zymography analysis indicated that DLK1 could affect both matrix metalloproteinase-9 (MMP9) expression and its extracellular activity. An analysis of NOTCH1 and HES1 gene expression and Notch intracellular domain (NICD) nuclear translocation during DLK1 stimulation or depletion demonstrated that DLK1 could activate Notch signaling in lung cancer cells. Additionally, the elevated expression of MMP9 induced by DLK1 stimulation could be significantly decreased by inhibiting Notch signaling using γ-secretase inhibitor (GSI). The data presented in this study suggest that DLK1 can promote the invasion of lung cancer cells by upregulating MMP9 expression, which depends on Notch signaling.