Abstract: Two isostructural metal organophosphonates, Cd［NH(CH_2PO3H)₃］(H_2O)₃ (compound 1) and Mn［NH(CH_2PO3H)₃］(H_2O)₃  (compound 2), were hydrothermally synthesized by using nitrilotris(methylenephosphonic acid), ［N(CH_2PO3H2)3］ (NTP), as a organic ligand. Single-crystal X-ray diffraction analysis revealed that compound 1 and compound 2 both crystallized in the monoclinic space group P₂(1)/c (No.14). Take the structure of compound 1 for example, two oxygen atoms from phosphonate of NTP chelated to one cadmium atom to form an eight-ring and one of them additionally interconnects the neighboring metal centre to form a one-dimensional (1D) zigzag chain parallel to the z-axis direction. The phosphonate groups of NTP and coordinate water molecules were both participated in hydrogen bonding. Further characterizations of compounds 1 and 2 have been performed, including X-ray powder diffraction, IR, ICP and CHN analyses.