One cadmium(II) coordination polymer [Cd(phen)(4,6-dimethyl-5-nitro-isophthalie acid)](n) (1) has been synthesized with 4,6-dimethyl-5-nitro-isophthalic acid, phen (phen = 10-phenanthroline), and cadmium nitrate in the mixture of DMSO:H2O (V:V = 2:1). Crystallographic data for this complex: monoclinic, space group P2(1)/c, with a = 11.668(2), b = 11.841(6), c = 14.446(2) angstrom, beta = 102.440(4), V = 1.9491(8) angstrom(3), D-c = 1.805 g/cm(3), Z = 1, F(000) = 1056, GOOF = 0.999, R = 0.0280 and wR = 0.0662. The crystal structure analysis indicates that the cadmium ion is coordinated with two nitrogen atoms of one 1,10-phenanthroline molecule and five oxygen atoms from three 4,6-dimethyl-5-nitro-isophthalic acid molecules. The curve of TGA shows that 1 is stable up to 380.0 degrees C.