Recent experiment showed that nano-fibers composite ceramics, fabricated through SHS process, could acquire high toughening and strengthening. Composite ceramics are mainly composed of fiber eutectic with random orientation, in which nanometer sized zirconia fibers are dispersed within the alumina matrix. First, it can be visualized that tensile force is transmitted from the matrix to the nano-fiber by means of shear stress that develop along the fiber- matrix interface. Then the shear stress on the surface of the fiber eutectic is obtained. It related to the volume fraction and slenderness ratio of the nano-fibers, and external strain of the fiber eutectic. The maximum shear stress is at the ends of the fiber eutectic perpendicular to applied tensile stress. As maximum shear stress on the surface of the fiber eutectic is equal to the critical shear stress, the composite ceramics would crack. So the applied tensile stress on composite ceramics arousing the surface crack of the fiber eutectic is gotten.