Abstract: Arginine-arginine-leucine(RRL) a tumor targeting peptide is designed to link the bifunctional chelator mercaptoacetyltriglycine(MAG₃), aiming to explore the labeling conditions of ⁹⁹Tc^m and to evaluate the stability of this probe in vitro. Then ⁹⁹Tc^m was labeled through the SnCl₂ reduction method, and the main factors affecting the labeling were investigated respectively to obtain the suitable labeling conditions. The paper chromatography was used to determine the labeling rate and radiochemical purity. As a result, the purity of MAG₃-RRL is 98.94%. Under the suitable labeling conditions, the labeling rate is 93.67%±1.10%, and the radiochemical purity is 94.32%±0.19%(n=3). When ⁹⁹Tc^m-MAG₃-RRL is placed in normal saline at room temperature and 50% BSA at 37 ℃, the radiochemical purity of ⁹⁹Tc^m-MAG₃-RRL is more than 90% (n=3) within 6 hours. The highest replacement rate of ⁹⁹Tc^m-MAG₃-RRL is 0.57%±0.21% in cysteine solution, and 0.41%±0.04% in normal saline control(n=3, P>0.05). The distribution coefficient lg P of ⁹⁹Tc^m-MAG₃-RRL is -0.15±0.01(n=3). In conclusion, the results show that MAG3 can successfully be connected to RRL polypeptide and further improve the labeling rate, and the preparation method of ⁹⁹Tc^m-MAG₃-RRL is simple and fast. ⁹⁹Tc^m-MAG₃-RRL probe has good stability in vitro, which provides a good basis for further biological experiments.