Small interfering RNA (siRNA), which induces sequence-dependent gene silencing, has been widely studied. We previously developed polycation liposomes (PCL) as carriers of plasmid DNA and succeeded in showing their potent gene expression efficiency. In the present study, we optimized PCL for siRNA transfection and used it to determine the role of Argonaute2 (Ago2), a main constitution protein of RNA-induced silencing complex (RISC), on angiogenesis. We determined the biological effect of Ago2 knockdown on the angiogenic potential of endothelial cells. Human umbilical vein endothelial cells (HUVECs) stimulated with cytokines including vascular endothelial growth factor (VEGF) were used as an in vitro angiogenesis model. Our data showed that Ago2 knockdown using polycation liposomal Ago2-siRNA (siAgo2) suppressed indispensable processes of angiogenesis, namely endothelial cell proliferation and tube formation. Furthermore, TUNEL staining indicated that the treatment with siAgo2 increased apoptotic cells in comparison to that with control siRNA. These results might be caused by disorder of microRNA system. Next, we attempted to construct systemic siRNA delivery system targeting to angiogenic vessels. Synthetic siRNA was incorporated in polycation liposomes modified with polyethyleneglycol (PEG) and the functional peptide on their surface. Peptide-modified liposomes enhanced cellular uptake of siRNA in comparison with non-modified liposomes. Thus APRPG-modified liposomal siAgo2 may be useful for tumor treatment.