Activity-regulated cytoskeleton-associated (Arc) protein, endogenously released in neurons, has been known for synaptic plasticity and memory formation. Recent discoveries have indicated that Arc protein interacts with RNAs and forms retrovirus-like capsids, which have the potential for mRNA-based therapeutics delivery. To apply to the mRNA drug delivery system, the study of the properties of the capsid is still limited. Here, we investigate the stability of Arc protein-formed retrovirus-like capsid and if engineering on mRNA can increase the stability of the capsid and therefore enhance the efficiency of mRNA loading. Dynamic light scattering (DLS) and nano tracking analysis (NTA) monitored the in-solution size and concentration changes of the particles over time, and transmission electron microscopy (TEM) visualized the capsid and its changes over time. With the addition of Arc 5’ untranslated region (UTR) motif, the stability of the retrovirus-like capsid formed from rat Arc (rArc) protein-mRNA interaction is observed to be enhanced.