A new selenidoarsenate [Co(en)(3)](2)As2Se5 1 (en = ethylenediamine) was synthesized by a reaction of AsCl3, CoCl2 center dot 6H(2)O and Sein en under mild solvothermal conditions, and the structure was determined by single-crystal X-ray diffraction. 1 crystallizes in the orthorhombic system, space group Pbca with a = 15.5092(12), b = 11.5756(9), c = 17.8138(14) angstrom, V = 3198.1(4) angstrom(3), M-r = 1023.12, Z = 4, D-c = 2.125 g/cm(3), mu = 8.807 mm(-1), F(000)' = 1976, S = 1.171, the final R = 0.0591 and wR = 0.1067 for 3170 observed reflections with I > 2 sigma(I). I consists of isolated dimeric [As2Se5](4-) anion constructed by two corner-sharing AsSe3 trigonal pyramids and transition metal [Co(en)(3)](2+) cations. The optical absorption property and thermal stability of 1 are reported.