A novel estimation for the critical size of the frontal process zone of ceramics is proposed using a single-edge V-notched beam (SEVNB) technique. A three-point flexure test is carried out on aluminum titanate ceramics containing a sharp V-shaped notch with different depth. An exact solution of the critical local stress is analyzed at a critical distance from the notch tip. The critical frontal process zone size is determined as the distance between the notch tip and the point where the critical local stress equals the flexural strength of specimens without notches, based on the local fracture criterion and the Griffith-Irwin criterion. The critical size of the frontal process zone, the fracture toughness and the flexural strength were also estimated for several materials, such as, alumina, porous alumina, and alumina-based nanocomposites. The relationship between these mechanical properties indicated that there was an almost linear relationship between the fracture toughness and the resultant of strength and square root of the critical frontal process zone size, and that both of them must be increased to improve the fracture toughness of ceramics.