Thermal lens effect was calculated for several rod-axis orientations. Due to the thermal strain photoelastic effect., there is a change of refractive indices in the [001] and the [211] rods. The change depends on the angles between the main axis of refractive index and the coordinate system. The thermally induced bi focus in these two directions is different from that of [111] rods. The former is a function of the angles, whereas the latter is not dependent on the angles. Relation between effective thermal focal length and input power measured experimentally are in good agreement with theory.