Abstract: A thermo-sensitive poly(N-isopropylacrylamide) (PNIPAAm) with terminal group DTPA was designed and synthesized, and the DTPA-PNIPAAm showed similar low critical solution temperature (LCST) to that of PNIPAAm. The preparation conditions of ¹⁵³Sm-DTPA-PNIPAAm, stability in vitroand biodistribution after intratumoral injection in tumorbearing mice were investigated. The experimental results of preparation investigation show that it is better to prepare ¹⁵³Sm-DTPA-PNIPAAm at pH=7-9 with 20-25 mg of DTPA-PNIPAAm and the reaction time more than 20 min. The labeling efficiency under the best reaction conditions is 93.4%. The labeled complex is stable in vitroand the radiochemical purity remains 96.5% after 76 h. After intratumoral injection, （83.2±9.7）% of the total radioactivity is kept at injected position after 72 h.