Using N,N'?Dimethylpiperazine as structure directing agent, a 3D open?framework neodymium oxalate [Nd(H2O)(C2O4)2](C6N2H16)0.5?2H2O (compound 1) was synthesized under hydrothermal condition. The single crystal X?ray diffraction reveals that the structure is composed of alternating single?capped tetragonal antiprism NdO9 and oxalic acid units connected to each other. The structure contains three?dimensional 12?membered ring channels, in which water molecules and protonated Me2ppz cations are located in the channels along the b and a axes, respectively. The presence of guest water molecules provides the structural basis for proton conductivity. The results of hydrothermal and acid?base stability tests show that the structure of compound 1 had excellent stability. AC impedance tests show that compound 1 has a high proton conductivity, which is 2.81×10-4 S/cm at 348 K and 98% RH.