A unified theory of the size effect on the yield stress of un-notched, notched and cracked specimens is developed on the basis of the yield zone growth curve, that shows the relation between the yield zone size and the applied stress (they are normalized by the adequately selected size parameter and the yield strength of material, respectively). The yield zone growth curve of the cracked specimen can be described by Dugdale model, and that of the un-notched specimen can be done by partially/fully plastic solution. However, the yield zone growth curve of the notched specimen has not been clear in spite of many studies. So, in the present report, an adequate size parameter for normalizing the yield zone growing at the notch root is considered, and Notch Root Yield Zone (NRYZ) model is devised and verified by elastic plastic FEM analyses. As a result, the yield zone growth curve of the notched specimen can be formulated from NRYZ model. The basic equation on size effect is derived from each yield zone growth curve of geometric similitude notch condition, and the size effect criterion is constituted based on the equation of geometric dissimilitude notch condition that is selected particularly for replacement, such as an hour-glass type notch (in place of an un-notch case), a spherical notch/small drill hole, a crack-like notch, etc.
