Branched poly(ester amine)s (PEAs) based on tris[2-(acryloyloxy)ethyl]isocyanurate (TAEI) cross-linked low molecular weight polyethylenimine (LPEI, Mw: 0.8k/1.2k/2.0k) polymers have been evaluated for their ability to deliver antisense 2′-O-methyl phosphorothioate RNA (2′-OMePS) through in vitro and in dystrophic mdx mice. Almost all PEAs improved exon-skipping of 2′-OMePS both in vitro and in vivo. Especially the PEAs composed of PEI 2.0k (C series) enhanced 2′-OMePS delivery more effectively than PEI 0.8k or 1.2k-based series using a GFP reporter-based C2C12 myoblast culture system and in mdx mice. The highest efficiency of targeted exon-skipping of 2′-OMePS was obtained by PEA C14 [TAEI-PEI 2.0k (1:4)] with up to 6 fold increase compared with 2′-OMePS only in mdx mice via intramuscular injection without increased muscle damage. Enhanced exon-skipping and lower toxicity highlight the potential of this series biodegradable PEAs as safe and efficient 2′-OMePS delivery vectors for treating diseases such as muscular dystrophy with splicing modulation.