A series of cationic amphiphlic copolymers (Z series) constructed from Tween 85 and low molecular weight (Mw) polyethyleneimine (LPEI) have been evaluated for the delivery of antisense 2'-O-methyl phosphorothioate RNA (2'-OMePS) in both cell culture and dystrophic mdx mice. All Z copolymers improved the 2'-OMePS-induced dystrophin expression both in vitro and in vivo compared with PEI 25k formulated or 2'-OMePS alone. The most effective polymers are in the order of Z9 > Z3 > Z7, Z1, Z2, Z6 > others by formulation at the dose of 20 μg mL-1 in myoblast cell culture. Significantly enhanced exon-skipping of 2'-OMePS with Z polymers in mdx mice was obtained in the order of Z7 > Z9, Z3 > Z8, Z6 > others. The highest efficiency of targeted exon-skipping with Z7 [T85-PEI 2k (1 : 1)] reached over 8 fold compared with 2'-OMePS alone in mdx mice. Further analyses of the structure and function indicate that the more hydrophobicity and lower PEI content of the polymer microstructure are, the greater are the delivery efficiency and exon-skipping. The unique hydrophobic interactions between the Z polymers and 2'-OMePS likely create more stable complexes in primarily hydrophilic environments both in vitro and in vivo. The overall results suggested that Tween 85 modified LPEIs provide a promising delivery approach for applications of 2'-OMePS oligonucleotides as therapeutic reagents.