Theoretical and experimental results are reported on the thermal lensing effect caused by the radial thermal gradient present in optically pumped Nd:YAG laser rods. The presented theory is in agreement with the experimental observations. The results reveal that a Nd: YAG rod under pumped light becomes a positive lens with two focal lengths. The temperature dependent variation of the refractive index constitutes the major contribution of the thermal lensing. The stress dependent variation of the refractive index modifies the focal length about 20%. The effect of end-face curvature caused by an elongation of the rod is less than 6%.