Two lanthanide-organic frameworks, {Ln(TDA)(1.5)(H2O)(2)}(n) [TDA = thiophene-2,5-dicarboxylic acid anion; Ln = Gd (1), Dy (2)], were structurally and magnetically characterized. The magnetic studies revealed that the ferromagnetic coupling existed between adjacent lanthanide ions in 1 and 2, and only 2 displays slow magnetic relaxation behavior with tau(0) = 2.4 x 10(-1) s and Delta E/k(B) = 44.2 K. To our knowledge, it is rather rare that ferromagnetic coupling and slow magnetic relaxation coexist in three-dimensional lanthanide-based frameworks.