Abstract: Bleomycin A5(BLMA5) is conjugated with cyclic DTPA anhydride and labeled with 153 Sm. The conditions of conjugation and labeling are investigated. The possible composition of BLMA5-DTPA is analyzed by measurement and computation. The results show that bleomycin A5 can conjugate with DTPA anhydride easily when the mole ratio of cDTPAA to BLMA5 is more than 10 in weak basic media, and the conjugating yield is more than 80%;A molecule of BLMA5 conjugate with 2 molecule of cDTPAA when the mole ratio of cDTPAA to BLMA5 is not less than 15, and the conjugative positions is on 2 different N atoms. The results of labeling experiments indicate that the labeling yield of 153 Sm-DTPAA-BLMA5 is 98% at pH7~9 and room temperature for 30 min when the mole ratio of DTPA-BLMA5 to 153 Sm is more than 8 153 Sm-BLMA5-DTPA is stable and the radiochemical purity is more than 96% at room temperature for 6 days.