Gene therapy using siRNA can be a new potent strategy to treat many incurable diseases, including cancer and viral infection, at the genetic level. Therapies using siRNA essentially need an efficient and safe method of siRNA delivery into cells while maintaining the stability of the siRNA. Here, we designed new fusion peptides of SPACE and oligo arginine. Fusion peptides formed uniform self-assembled nano-complexes without additional reactions. Moreover, siRNAs were stable in nano-complexes for four days in 10% fetal bovine serum. Cellular uptake efficiency of each complex was similar or higher than that of commercialized available Lipofectamine™ 2000. GAPDH-siRNA/peptide complex knock GAPDH mRNA down similar to that mediated by Lipofectamine™ 2000. The increase of arginine residues in fusion peptides induced higher siRNA stability, which enhanced GAPDH knockdown. Co-localization and cellular internalization of siRNA/S-R15 complexes were verified peripherally around the nucleus. The endocytosis pathway of the siRNA/S-R15 complex was identified via lipid-raft mediated endocytosis. Besides, the biosafety of each fusion peptide was proven under a particular concentration. Therefore, a safe and stable self-assembled complex could be expected to deliver siRNA into cells efficiently for the treatment of many diseases.