The title complex has been synthesized in acetonitrilesolution under N2 protection, and its crystal structure has been determined by X-ray diffraction method. The crystal belongs to monoclinic space group P21/a. The cell parameters are a=1.7335(4), =1.8027(4), c=0.8772(3)nm, β=97.64(3)°, V=2.7171(2)nm^3, Z=4. The central ion Er^3^+ is coordinated by four nitrogen atoms from 1, 4, 7, 10- tetraazacyclododecane (Cyclen), three oxygen atoms of three CF3SO3^- and a nitrogen atom of a acetonitrile molecule. The coordination number is eight and the complex takes a coordination polyhedron of square antiprism. The bonds between Er^3^+ and O is much stronger than Er-N (Cyclen) and Er-N (acetonitrile), and their average bond lengths are 0.2316nm, 0.2468nm and 0.2510nm, respectively.

Key words: ELEMENTAL ANALYSIS, CRYSTAL STRUCTURE, X-RAY DIFFRACTION ANALYSIS, NITROGEN HETEROCYCLICS, FOUR-CIRCLE DIFFRACTOMETER, ERBIUM COMPLEX