The purpose of the present investigation is to examine the effect of plate thickness on the ductile fracture toughness values experimentally and numerically. Considerable attention is paid to the stress, strain and deformation in the vicinity of the crack tip in order to analyze the fracture behavior. Fracture toughness tests are performed for center cracked specimens (CCS) of SUS430 stainless steel and compact tension specimens (CTS) of SS41 mild steel. The thicknesses of CCS are 0.4, 0.7 and 1.2 mm and those of CTS are 1, 2, 5, 10 and 19 mm. The deformation, stress and strain in the vicinity of the crack tip of CCS are obtained numerically by using 3-dimensional elastic-plastic Finite Element Method. In addition, the authors propose the method to evaluate the critical J -integral value at the onset of crack extension by using the numerical results of equivalent plastic strain in the vicinity of the crack tip. The results in this study are summarized as follows : 1) At thinner plate thickness than 5 mm, the ductile fracture toughness ( J_i ) values decrease with in an decreasing plate thickness for the metallic materials used in this study. 2) Even very thin plate thickness, the stress condition at the crack tip becomes high triaxiality due to plastic constraint. 3) Estimated values of J_i , which are evaluated by using the equivalent plastic strains, displayed good agreement with the experimental results. Then, it is considered that the critical value of the equivalent plastic strain is one of the fracture parameters of ductile metals at the crack initiation.