A novel siRNA delivery system based on polyelectrolyte complex (PEC) micelles was introduced in this study. Vascular endothelial growth factor (VEGF) siRNA was conjugated to poly(ethylene glycol) (PEG) via a disulfide linkage (siRNA-PEG). The siRNA-PEG conjugate could form PEC micelles by interacting with cationic polyethylenimine (PEI) as a core forming agent. The VEGF siRNA-PEG/PEI PEC micelles showed greater stability than naked VEGF siRNA against enzymatic degradation. Under a reductive condition similar to cytosolic environment, an intact form of siRNA was released from the siRNA-PEG conjugate by cleavage of the disulfide linkage. The VEGF siRNA-PEG/PEI PEC micelles effectively silenced VEGF gene expression in prostate carcinoma cells (PC-3) up to 96.5% under an optimized formulation condition. They also showed a far superior VEGF gene silencing effect than VEGF siRNA/PEI complexes even in the presence of serum. This study suggests that the siRNA delivery system using VEGF siRNA-PEG/PEI PEC micelles could be potentially applied to RNAi-based anti-angiogenic treatment of cancer in vivo.