The present invention is related to hybrid conjugates formed by covalently bonding siRNA (small interfering RNA) molecules to hydrophilic polymers for improving stability of the siRNA molecules effective for delivering the siRNA in vivo, and polyelectrolyte complex micelles formed by ionic interactions between the conjugates and multifunctional cationic compounds. The siRNA-hydrophilic polymer conjugates and polyelectrolyte complex micelles derived therefrom can be used for improving stability of the siRNA molecules in vivo. Consequently, the delivery of siRNA molecules for therapeutic applications into cells can be facilitated, and the siRNA is still active even though a small dose of the siRNA is used.