Undoped, 5 and 7mol% MgO doped LiNbO3 single crystals were grown by conventional Czochralski technique. Comparative study of the nonlinear refractive index and nonlinear absorption coefficient in these crystals was undertaken using Z-scan technique. We have also calculated real and imaginary parts of the third-order nonlinear susceptibility using experimentally obtained values of nonlinear refractive index and nonlinear absorption coefficient. We found the change in sign of the real part of the susceptibility by doping the LiNbO3 crystal while the imaginary part of susceptibility is found to minimize at 5 mol% doping concentration. The absolute values of the third-order susceptibility decreases with increasing doping concentration. (C) 2009 Elsevier B.V. All rights reserved.