We synthesized functional retinoic acid (RA)-polyethyleneimine (PEI) complex nanoparticles. NH groups of branched PEI chains were electrostatically interacted with carboxyl groups of RA surfaces to form cationic RA-PEI complex nanoparticles. We observed that the average diameter of RA-PEI complex nanoparticles was approximately 70 nm and the morphology of complex nanoparticles was homogeneous circular shape. To confirm the synthesis of RA-PEI complex nanoparticles, we characterized complex nanoparticles using (1)H nuclear magnetic resonance (NMR), indicating that hydrophilic branched PEI chains were covered on hydrophobic RA surfaces. Furthermore, we demonstrated that pH enabled the control of amounts of RA released from RA-PEI complex nanoparticles, showing that RA exposed to acidic pH 5 was steadily released (∼76%) from complex nanoparticles, whereas RA was rapidly released (∼97%) at pH 7.4 on day 11. We also observed that RA-PEI complex nanoparticles induced embryonic stem (ES) cell-derived neuronal differentiation. Therefore, this RA-PEI complex nanoparticle is a potentially powerful tool for directing murine ES cell fate.