Over the past decades, there is an increasing demand for developing a smart targeted cancer therapy to deliver chemotherapeutic drugs directly to cancer cells while minimizing the killing of healthy noncancerous cells. The purpose of this study is to develop drug encapsulated nanoparticles that can be specifically taken up by the prostate cancer cells and release their encapsulated chemotherapeutic drugs over an extended period to achieve a clinical response. The targeted nanoparticles were developed using an amphiphilic block copolymer containing the following three segments 1) a targeting molecule made of a ribonucleic acid (RNA) aptamer that can bind specifically to the human prostate cancer cells, 2) a segment of poly(ethylene glycol) (PEG) to maximize the nanoparticle circulation half life, 3) a segment of poly(D,L-lactide-co-glycolide) (PLGA) for encapsulation and sustained release of chemotherapeutic drugs. As a proof of concept, we synthesized an aptamer-PEG-PEG triblock copolymer using PLGA with carboxylic acid terminal group, a heterobifunctional PEG, (amine-PEG-carboxylic acid) and an amine modified A10 prostate specific membrane antigen (PSMA) aptamer. The synthesis reaction was carried out in two steps. 1) synthesis of PLGA-PEG diblock polymer by conjugating of the carboxylic end of PLGA to the amine terminal of a bifunctional PEG; and 2) conjugation of the carboxylic acid end of the bifunctional PEG to the amine modified aptamer. The nanoparticles were prepared by precipitation of triblock copolymer in aqueous solution. The size and surface charge of the nanoparticles were found to be 185 nm and -26, respectively. To demonstrate prostate cancer cell specific uptake, nanoparticles were encapsulated with radiolabeled paclitaxel. The percentage of nanoparticles endocytosed by the cells was quantified by the amount of radioactive paclitaxel detected. In the case of PC3 prostate epithelial cells, which do not express the PSMA protein, no measurable difference in binding was observed between the targeted aptamer-PEG-PLGA and untargeted PEG-PLGA nanoparticles. In LNCap cells, which do express PSMA protein, the data demonstrate significant enhancement in the binding of targeted nanoparticles vs. the non-targeted nanoparticles. It was also found that the nontargeted nanoparticles showed very low binding efficiency to both PC3 and LNCaP cells, presumably attributed to the presence of PEG group. We also evaluated the nanoparticle specific uptake by the prostate cancer cells in vivo using a xenograft model of LNCaP tumors developed by s.c. injection in the flank of BALB/c nude mice. The results showed that systemic administration of aptamer-triblock nanoparticles had significant tumor targeting to the LNCap tumor compared with the nontargeted nanoparticles. Both in vitro and in vivo biodistribution results demonstrated cancer cell specific targeting using the aptamer-PEG-PLGA triblock nanoparticles. The formulation of such polymer can be further developed as a platform technology for cancer therapeutic and diagnostic applications.