To further enhance the transfection efficiency of a micelleplex system based on monomethoxy poly(ethylene glycol)-block-poly(ε-caprolactone)-block-poly(L-lysine) (MPEG-b-PCL-b-PLL), cholesterol (Chol) moieties are attached to the ε-termini of PLL segments to obtain MPEG-b-PCL-b-PLL/Chol. The structure and morphology of the copolymer are studied by 1H-NMR, TEM and DLS (dynamic light scattering). The cytotoxicity, cell uptake, endosomal release and mRNA knockdown are studied by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay, flow cytometry, CLSM (confocal laser scanning microscopy) and RT-PCR (real-time polymerase chain reaction). The results show that compared to their precursor MPEG-b-PCL-b-PLL, the cholesterol-grafted copolymer shows significantly lower toxicity, more rapid cellular endocytosis and endosome escape, and consequently displays enhanced siRNA transfection efficiency even at a lower N/P ratio. These improvements are ascribed to enhanced interaction of the cholesterol moieties with both cellular membrane and endosomal membrane. Moreover, effect of the PLL block length is examined. The final conclusion is that long enough PLL segments and incorporation of proper fraction of cholesterol onto the PLL segments benefit the enhancement of siRNA transfection efficiency. To further enhance the transfection efficiency of a micelleplex system based on monomethoxy poly (ethylene glycol) -block-poly (ε-caprolactone) -block-poly (L-lysine) (MPEG-b-PCL- Chol) moieties are attached to the ε-termini of PLL segments to obtain MPEG-b-PCL-b-PLL / Chol. The structure and morphology of the copolymer were studied by 1H-NMR, TEM and DLS (dynamic light scattering). The cytotoxicity, cell uptake, endosomal release and mRNA knockdown were studied by MTT (3- [4,5-dimethylthiazol-2-yl] -2,5-diphenyltetrazolium bromide assay, flow cytometry, CLSM The results show that compared to their precursor MPEG-b-PCL-b-PLL, the cholesterol-grafted copolymer shows significantly lower toxicity, more rapid cellular endocytosis and endosome escape, and displays enhanced siRNA transfection efficiency even at a lower N / P ratio. These improvements are ascribed to enhanced interact ion of the cholesterol moieties with both cellular membrane and endosomal membrane. Moreover, the effect of the PLL block length is examined. The final conclusion is that long enough PLL segments and incorporation of proper fractions of cholesterol onto the PLL segments benefit the enhancement of siRNA transfection efficiency.