Thermal lens effects are severe for Nd:GdVO4 laser crystals with a end-pumping scheme. A 532 nm laser is used as a probe beam in experiments. By detection of the far-field image of the probe beam,the thermal effect could be measured. A model is presented using beam interference theory. Based on this model,the phenomenon at far-field is explained and a new formula for calculating the thermal focal length is formed. Thermal lens measurements are also performed using a resonator stability method. The results agree well with the beam interference model and the thermal focal length formula.