Notch signaling regulates cell-fate decisions during development and postnatal life. Little is known, however, about the role of Dll4-Notch signaling between cancer cells, and how it affects cancer metastasis. We therefore assessed role of Dll4-Notch signaling in cancer metastasis. We generated a soluble Dll4 fused to the human IgG1 Fc constant region (Dll4-Fc) that acts as a blocker of Dll4-Notch interactions, and introduced it into human SCLC cell lines expressing high level (SBC-3 and H1048) and low level (SBC-5) of Dll4. The effect of Dll4-Fc on metastasis of SCLC was evaluated using a mouse model. Although Dll4-Fc has no effect on liver metastasis of SBC-5, the number of macroscopic metastatic nodules in the livers of mice inoculated with SBC-3 and H1048 cells expressing Dll4-Fc were significantly lower than that in the livers of mice injected with control cells. In order to study the molecular mechanism of the effect of Dll4-Fc on liver metastasis, PCR array analysis was performed. Since expression of NFkB target genes were affected by Dll4-Fc, we explored whether NFkB is involved in Dll4-Notch signaling in the cancer cells. We observed by electrophoretic mobility shift assay that both with and without TNF-a-stimulated NFkB activities were down-regulated in Dll4-Fc-overexpressing SBC-3 and H1048 cells, compared with control cells. Moreover, interaction of Notch1 intracellular domain with p65 enhanced the NFkB signaling. These results suggest that Dll4-Notch signaling in cancer cells plays a critical role in liver metastasis of small cell lung cancer by regulating the NFkB signaling.