〔Abstract〕 Objective To develop an octreotide(Oct) functionalized polymeric nanosized contrast agent for targeted magnetic resonance(MR) imaging of tumor. Methods Reversible addition-fragmentation chain transfer(RAFT) polymerization had been employed to synthesize branched copolymer nanoparticles composing n-butyl methacrylate and ethylene glycol dimethacrylate copolymer segments as the core, and oligoethylene glycol methacrylate together with segment of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid as the shell respectively, followed by the modification of Oct as corona. These polymeric nanoparticles were chelated with gadolinium ions to obtain a novel tumor-targeted branched copolymer nanosized MR contrast agent [P(DO3 A-Gd)-Oct]. The morphology of the products was analyzed by nuclear magnetic resonance(NMR), dynamic light scattering(DLS) and transmission electron microscope(TEM). The stability, hemolysis rate, cytotoxicity andin vivosafety of P(DO3 A-Gd)-OCT were preliminarily tested. Magnetic resonance relaxation rates were measured using a clinical 3.0 T magnetic resonance imager andin vivomagnetic resonance imaging was performed in H22 hepatoma bearing mice. Results According to the DLS and TEM results, the hydrodynamic diameter of P(DO3 A-Gd)-Oct was about 20 nm. After the exposure to P(DO3 A-Gd)-Oct, the hemolysis rate was only 0.34 %, and no obvious cyto-toxicity was observed. Compared with normal mice without any treatment, there was no obvious change of the pathological structures of major organs or the major blood biochemical indexes after the intravenous(i.v.) injection of P(DO3 A-Gd)-Oct. The relaxivity of P(DO3 A-Gd)-Oct(r1) was 8.33 mM-1s-1. After 60 min and 120 min of the i.v. injection, there were obvious increase of the signal in the tumor region in T1 weighted images of H22 tumor bearing mice, which was attributed to the increased concentration of gadolinium ions in tumor tissues compared with the non-targeted control. Conclusion P(DO3 A-Gd)-Oct exhibits bio-compatibility and excellent MR contrast ability, which can contribute to the enhancement of the sensitivity and accuracy in the diagnosis of malignant tumors such as liver tumor.