Messenger RNA (mRNA) has high potential for developing a wide range of therapeutics, though effective delivery systems are still required for its broad application. Polymeric micelles loading mRNA via polyion complexation with block catiomers are emerging as promising carriers for mRNA delivery. However, the in vivo stability of polyion complexes (PIC) has been limited so far, demanding for stabilization strategies. Controlling the stiffness of the cationic segment in the catiomers could promote PIC formation and micelle stability. Nevertheless, the impact of polycation flexibility on the function of mRNA-loaded PIC micelles (mRNA/m) remains unknown. Herein, we studied the effect of the stiffness of the polycation segment of catiomers on the association and performance of mRNA/m toward enhancing stability and delivery efficiency. Thus, a block catiomer system having polycation segments with different flexibility, i.e. poly(ethylene glycol)-poly(glycidylbutylamine) (PEG-PGBA) and PEG-poly(L-lysine) (PEG-PLL), was developed and used to prepare mRNA/m. The flexible PEG-PGBA catiomer allowed 100-fold stronger binding to mRNA than PEG-PLL, resulting in mRNA/m with enhanced mRNA protection against enzymatic attack and resistance to dissociation from polyanions. mRNA/m from PEG-PGBA significantly increased translation efficiency both in vitro and in vivo, and enhanced mRNA bioavailability in blood after intravenous injection. These results indicate the importance of polycation flexibility for developing stable PIC with mRNA directed to enhance delivery.