While using a laser to process hard tissue, it is difficult to guarantee, that the laser beam is always perpendicular to the tissue surface. Therefore, it is necessary to know the dependence of ablation depth on angle of incidence for preoperative planning. Considering the propagation of the ablation front, an Addition Model is developed in this work. It indicates that the shape of a crater ablated by a single pulse with non-zero angle of incidence can be regarded as the sum of the original tissue surface and a symmetric profile, which is corresponding to the shape of a crater ablated by perpendicularly incident beam. Meanwhile, the ablation depth at a point P is defined as the distance from P to the original tissue surface along the optical axis of the incident beam. In the context of this definition, the dependence is experimentally studied. The results of the experiments were unexpected: the ablation depth is independent of angle of incidence up to ca. 55°. Possible reasons for these results are discussed.