Our-own-N-layered integrated molecular orbital + molecular mechanics (ONIOM) method has been applied to calculate C-13 chemical shift shielding tensor of the carboxyl carbon in three amino acid crystals. The calculated shielding tensors are in good agreement with the experimental values. The root mean square (RMS) has dramatically decreased to 5.9 ppm compared to that (35.5 ppm) previously reported in reference. The present investigation has proved that the surrounding lattice environment plays an important role in the calculation of chemical shift tensors of amino acids. (C) 2004 Elsevier B.V. All rights reserved.