In the present study, we synthesized a novel cationic copolymer comprised of polyethylene glycol 5000 (PEG5K), vitamin E (VE) and diethylenetriamine (DET) at 1:4:20 molar ratio. The resulting PEG5K-VE4-DET20 copolymer formed nanoassemblies when mixed with the neutral PEG5K-VE4 copolymer at 1:8 weight ratio, which were investigated as the nanocarriers for combined delivery of paclitaxel and let-7b mimic. We found that the PEG5K-VE4-DET20 nanoassemblies could entrap paclitaxel for an extended period and burst release the drug in the presence of cathepsin B, demonstrating the biodegradability of the copolymers. At N/P ratio of 12:1, the PEG5K-VE4-DET20 nanoassemblies formed stable polyplexes with let-7b mimic, which were efficiently taken up by tumor cells and underwent endosomal escape. In non-small cell lung cancer A549 cells that harbor mutant KRAS, paclitaxel and let-7b mimic-dual loaded nanoassemblies (N-PTX/let-7b) markedly potentiated the cytotoxicity of paclitaxel, induced apoptosis and diminished the invasiveness of tumor cells. In mice bearing subcutaneous A549 xenografts, intravenous administration of N-PTX/let-7b retarded tumor growth more efficaciously than Taxol. Our study demonstrates the promise of the PEG5K-VE4-DET20 nanoassemblies for concurrent delivery of hydrophobic drugs and miRNA mimics.