In this paper, the title complex K[In Ⅲ(Hdtpa)]·3.5H 2O(dtpa=diethylenetriaminepentaacetic acid) was synthesized and its crystal structure was determined by single crystal X ray structure analysis. The crystal belongs to monoclinic system, P 2 1/ c (No.14) space group with a =0.978 1(2) nm, b =0.733 8(1) nm , c =2.962 9(6) nm, β =91.81(3)°, V =0.212 6(1) nm 3, Z =4, M =606.31, D x=1.895 g·cm -3 , μ =1.382 1 mm -1 , F (000)=1 228, R =0.035 and R w=0.041 for 2 802 unique reflections. The complex anion [In Ⅲ(Hdtpa)] - has a pseudo monocapped trigonal prismatic seven coordination structure in which the seven coordinate atoms, three N and four O atoms, are all from a ligand dtpa. In addition, it can be seen that the complex anion [In Ⅲ(Hdtpa)] - can furnish a free carboxyl group(—CH 2COOH) being used for molecular embellishment to form the target diagnosis and treat drug of anticancer. [WT5HZ]