The structural nature of the nonlinear optical properties of LiB3O5 is analyzed using single-crystal nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR. The 3-coordinated trigonal [B(1) and B(2)] and 4-coordinated tetragonal [B(3)] sites are distinguished using the spectrum and the spin-lattice relaxation time in rotating frame T-1 rho, which was obtained from the B-11 MAS NMR. Moreover, the T-1 and T-1 rho values for Li-7 and B-11 are compared, and the activation energies were obtained. The T-1 rho values of the boron nuclei in LiB3O5 show no significant changes. These results may be closely related to the largest second-order nonlinear optical coefficient. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.