Pulmonary delivery of messenger RNA (mRNA) has huge potential for the treatment of a range of lung diseases. Inhaled dry powder formulation of mRNA is particularly attractive because it could improve the stability of the nucleic acids and the dry powder inhalers are relatively easy to use. A safe and effective mRNA delivery vector as well as a suitable particle engineering method are required for the production of inhalable mRNA powder formulations. To address these issues, we developed a new mRNA vector by modifying the KL4 peptide, which was previously investigated for RNA delivery. The PEG12KL4 peptide could form complexes with the mRNA and we successfully formulated them into dry powder by a spray freeze drying (SFD) technique. The SFD powder exhibited satisfactory aerosol properties for inhalation, with fine particle fraction (FPF) over 60%. More importantly, the biological activity of the PEG12KL4 /mRNA complexes were successfully preserved after drying as demonstrated in both in vitro and in vivo transfection studies using luciferase mRNA. Both the liquid and powder aerosol of PEG12KL4 /mRNA complexes achieved high luciferase expression in the deep lung region at 24 h following intratracheal administration in mice.