The ductile/brittle transition condition in an indentation of brittle materials is derived to utilize the prediction of the ductile mode abrading conditions. By modeling the action of an abrasive as a semi? spherical indentation, the ductile/brittle transition condition is represented by a critical radius (ac) of a spherical indenter: at a larger radius than that, a penny-shaped pre-existing crack extends as a median crack in the material.The stress intensity factors (KIC's) of the penny-shaped pre-existing cracks which are located at various depths with radii (c's) under the indentation are analyzed by using the stress fields derived by Chiang et al. The crack length-dependent critical stress intensity factor is applied to the evaluation of crack extension.The critical radii (ac's) of typical structural ceramics such as silicon nitride, silicon carbide, and alumina are deduced to be from 0.5-2.5μm. Although these materials have wide ranged characteristic constants such as Young's modulus, hardness (Hv), fracture toughness (KIC), Poisson's ratio, and mean grain diameter (dg). ac is given by the following rather simple equation under the assumption that the size of the pre-existing crack is the same as that of the mean crystalline grain.ac/dg=0.075·103.7KIC/(Hv√dg)The critical radius ac, which is experimentally evaluated in the view of preventing the bending strength deterioration of indented Si3N4 specimens, is fairly coincident with the analytical one.