Recognizing the "leaky" nature of ZIF-8, this approach turned towards a ZIF-8-based polymer composite mRNA delivery system, which involves initially complexing mRNA with polyethyleneimine (PEI) in a 2-methyl imidazole (2-MeIM) precursor solution, followed by zinc addition and encapsulation within ZIF-8 to form mRNA-PEI@ZIF-8 (mRPZ) nanoparticles (Scheme 1A). Extensive characterization of the mRPZ nanoparticles confirms the encapsulation of complexed mRNA within ZIF-8. This robust polymer complex core-MOF shell particle effectively retains and delivers mRNA, producing eGFP expression in vitro (Scheme 1B) and firefly luciferase expression in vivo (Scheme 1C). Comparative analyses were performed against commercial transfection reagents and mRNA-PEI alone. Remarkably, our findings demonstrate improved performance over mRNA-PEI, delayed expression kinetics, and the stabilization of mRNA complexes within mRPZ nanoparticles, with viable transfection maintained even after 3 months of room-temperature storage in vitro and 1 month in vivo.