The EPR spectra of Cu2+ in Sodium hydrogen oxalate monohydrate, NaHC2O4,H2O (SHOMH hereafter) single crystal was studied at room temperature. The angular variation of EPR spectra showed that the Cu2+ ion in SHOMH single crystal substitutes with Na+ monovalent cation together with a monovalent vacancy to compensate oxygen in the crystal. Since the crystal symmetry is triclinic, only one site is observed in the EPR spectra in three perpendicular axis. The spin Hamiltonian parameters were obtained, and the ground state wave function of Cu2+ ion in the lattice was constructed. Copyright © 1999 by Marcel Dekker, Inc.