We performed the temperature measurements of transport and non-linear optical properties, particularly optical third harmonic generation of PbTe and Gd-doped PbTe to elucidate the effect of the rare earth ion doping on behavior of the thermal conductivity and the third-order non-linear optical susceptibility. The feature of Gd-doped PbTe shows the existence of small values of the low temperature resistivities, (10(-6) Omega cm) and a very significant value of mobility mu = 1.5* 10(6) cm(2) V-1 s(-1). The thermal conductivity kappa decreases with incorporation of the rare earth ion (Gd) in PbTe matrice. The optical third harmonic generation (THG) shows that the great contribution of the phonons observed for PbGdTe compared to the PbTe, enhances the lattice thermal conductivity of network (k(ph)).