Small interfering RNA (siRNA) has great potential for the treatment of various respiratory diseases through RNA interference (RNAi), but their clinical application is hindered by the lack of a safe and effective pulmonary delivery system. KL4 peptide is a synthetic amphipathic peptide that was previously developed to mimic the function of pulmonary surfactant protein B (SP-B). Its potential as siRNA carrier for pulmonary delivery was examined in this study. The cationic KL4 peptide was able to bind with siRNA to form complexes at 15:1 ratio (peptide to siRNA weight ratio) or above. It also mediated efficient gene silencing on lung epithelial cells, with 20:1 ratio as the optimal ratio for siRNA transfection. Furthermore, the KL4/siRNA complexes were not toxic at concentrations used for transfection in vitro. The study shows that KL4 peptide appears to be a promising candidate for siRNA delivery. Further investigation on animal study and work on aerosol formulation are required to develop KL4 peptide as siRNA carrier for clinical application.