The LiNbO(3) congruent crystals doped with small Nd concentrations, <1 mol% Nd, and co-doped with Mg ions, 0-9 mol% Mg, were systematically investigated by means of micro-Raman spectroscopy in the Y and Z crystal directions. Results obtained from an undoped congruent crystal, an Nd-doped crystal, a Mg-doped crystal and Nd, Mg-co-doped crystals are compared. From the analyses of the results obtained in the Y direction, the Nd and Mg content dependence of the two lowest-Raman A(1)(TO(1)) and A(1)(TO(2)) modes, the half-width composition and the area ratio of the A(1)(TO(4)) and E(TO(8)) bands, we reached several conclusions about the incorporation mechanism of the Nd and Mg ions into the LiNbO(3) lattice. Likewise the Raman shift and half-width of the E(TO(1)) and E(TO(7)) modes were investigated in the Z direction. Results indicate that Mg and Nd ions are located in the Li site for low doping concentrations and for larger concentrations there is a replacement in both Li and Nb ion sites.