Novel redox-responsive amphiphilic cationic multi-block copolymers PEG(2000)-PLA(3000)-PEI1200-PLA(3000)-PEG(2000) and PEG(2000)-PLA(3000)-PEI1800-PLA(3000)-PEG(2000) were synthesized and self-assembled into micelles for co-delivery of siRNA and hydrophobic doxorubicin (DOX). The chemical structure and molecular weight of the copolymers were characterized by H-1 nuclear magnetic resonance and gel permeation chromatography, respectively. The copolymeric micelles were examined by dynamic light scattering, and their size, zeta potential and critical micelle concentration were determined. The in vitro drug release analyses indicated that reductive environment can trigger the release of DOX and siRNA by breaking the micelles. MTT assay demonstrated that the DOX/siRNA-loaded micelles are capable of inhibiting proliferation of SGC7901 cells. The results of fluorescence microscopy and flow cytometry verify the simultaneous delivery of DOX and siRNA from the nanomicellar particles into SGC7901 cells. The reduction-responsive cationic copolymers will provide a platform for constructing drug/gene delivery system toward cancer therapy.