Fine-tuning of chemical structures of polycation-based carriers (polyplexes) is an attractive strategy for safe and efficient mRNA transfaction. Here, mRNA polyplexes comprising N-substituted polyaspartamides with varied numbers of side chain aminoethylene repeats were constructed, and their transfection ability against human hepatoma cells was examined. Transfection efficacy clearly correlated with the number of aminoethylene repeats: polyplexes with odd number repeats (PA-Os) produced sustained increases in mRNA expression compared with those with even number repeats (PA-Es). This predominant efficacy of PA-Os over PA-Es was contradictory to our previous findings for pDNA polyplexes prepared from the same N-substituted polyaspartamides, that is, PA-Es revealed superior transfection efficacy of pDNA than PA-Os. Intracellular FRET analysis using flow cytometry and polyplex tracking under confocal laser scanning microscopy revealed that overall transfection efficacy was determined through the balance between endosomal escaping capability and stability of translocated mRNA in cytoplasm. PA-Es efficiently transported mRNA into the cytoplasm. However, their poor cytoplasmic stability led to facile degradation of mRNA, resulting in a less durable pattern of transfection. Alternatively, PA-Os with limited capability of endosomal escape eventually protect mRNA in the cytoplasm to induce sustainable mRNA expression. Higher cytoplasmic stability of pDNA compared to mRNA may shift the limiting step in transfection from cytoplasmic stability to endosomal escape capacity, thereby giving an opposite odd-even effect in transfection efficacy. Endosomal escaping capability and nuclease stability of polyplexes are correlated with the modulated protonation behavior in aminoethylene repeats responding to pH, appealing the substantial importance of chemistry to design polycation structures for promoted mRNA transfection.