The effects of lysine peptide lengths on DNA and siRNA packaging and delivery were studied using four linear oligolysine peptides with 8 (K8), 16 (K16), 24 (K24) and 32 (K32) lysines. Oligolysine peptides with 16 lysines or longer were effective for stable monodisperse particle formation and optimal transfection efficiency with plasmid DNA (pDNA) but K8 formulations were less stable under anionic heparin challenge and consequently displayed poor transfection efficiency. However, here we show that the oligolysines were not able to package siRNA to form stable complexes, and consequently siRNA transfection was unsuccessful. These results indicate that the physical structure and length of cationic peptides, and their charge ratios are critical parameters for stable particle formation with pDNA and siRNA and that without packaging, delivery and transfection cannot be achieved. This article is protected by copyright. All rights reserved.