The objective of this study is to develop a new type of cationic nanoparticles for the intracellular drug delivery to breast cancer. Poly(ester amine) (PEA) based on polyethylenimine and polycaprolactone was synthesized to make cationic PEA nanoparticles for all-trans retinoic acid (RA). In the 1H-NMR study, the proton signals of RA appeared in the spectrum of RA-loaded PEA nanoparticles in CDCL3, whereas they disappeared in D2O, suggesting that hydrophobic inner-core with hydrophilic outer-shell formed in water. RA release was faster at lower drug content and RA was released over a period of 20 days. RA-loaded PEA nanoparticles showed enhanced cytotoxicity compared with RA itself, whereas nanoparticles of PEA themselves did not show it. These results indicated that the cationic PEA provided an efficient intracellular delivery of RA.