In order to investigate the validity on a size requirement for K_Ic testing in the proposed revision of ISO12135, W-a≥1. 1 (K_Q⁄σ_ys)² , and on that in the present standard of ISO12135, W-a, a, B≥2. 5 (K_Q⁄σ_ys)², we analytically investigated the effect of plasticity at the crack tip on a provisional fracture toughness K_Q calculated from the force, at which a force-displacement curve (F-V curve) intersects a secant line given in accordance with the above standard. The nonlinear component of the F-V curve is the sum of two parts, one due to the plasticity at the crack tip and the other to actual crack extension. 3-dimensional elastic-plastic finite element analyses of CT specimens with varying sizes have been performed to calculate K_Qpl, which means K_Q obtained as a result of the plasticity at the crack tip without the crack extension. In the case of standard specimen (W⁄B=2, a⁄W=0. 5) , the correlation between K_Qpl and ligament size (W-a) was approximately given by W-a=1. 5 K_Q⁄σ_ys)² . It was found that the present standard leads to a plane strain fracture toughness K_Ic with a crack extension more than about 1%of the initial crack length, provided that W-a≥2. 5 (K_Q⁄σ_ys)² . Meanwhile, in the case of the proposed revision of ISO12135, it was shown to recognize K_Qpl as K_Ic, although the size requirement of W-a≥1. 1 (K_Q⁄σ_ys)² was met.