Two fumarato-bridged Co(II) coordination polymers Co(H 2 O) 4 L 1 and [Co 3 (H 2 O) 4 (OH) 2 L 2 ].2H 2 O 2 with H 2 L HOOCCH CHCOOH were prepared. Complex 1 consists of polymeric chains ~ 1 [Co(H 2 O) 4 (C 4 H 2 O 4 ) 2 / 2 ], which result from octahedrally coordinated Co atoms bridged by bis-monodentate fumarate anions and are assembled by interchain hydrogen bonds. Within 2, the edge-shared Co 2 O 1 0 bi-octahedra are connected to the CoO 6 octahedra to form 1D cobalt oxide chains and 3D open framework generated from the chains inter-linked by bis-bidentate fumarate anions displays rhombic tunnels, which are filled with the lattice H 2 O molecules. Thermal and magnetic behaviors of both the title coordination polymers are discussed. Crystal data: (1) monoclinic, P2 1 /c, Z=4, a=7.493(1)A, b=14.377(1)A, c=7.708(1)A, β=99.54(1) o , V=818.9(2)A 3 , R 1 =0.0304, and wR 2 =0.0669 for 1487 observed reflections (I>=2σ(I)) out of 1877 unique reflections; (2) monoclinic, P2 1 /c, Z=2, a=6.618(1)A, b=8.172(2)A, c=15.578(3)A, β=96.30(3) o , V=837.4(3)A 3 , R 1 =0.0360 and wR 2 =0.0663 for 1442 observed reflections (I>=2σ(I)) out of 1927 unique reflections.