Abstract: By using a type of single specimen method for determining ductile fracture toughness based on the load separation principle,the normalization method and the load separation parameter S_Pb method were investigated with a steam turbine rotator steel Cr2Ni2MoV.It was found that in the normalization method the blunt line equation used to blunt correction had an influence on the determination of J resistance curves within a limit crack extension.A modified separation parameter S_Pb method was proposed to eliminate the effect of reference blunt cracked specimens on the original separation parameter S_Pb method.The blunt corrected initial crack length and final crack length were recommended to use as two calibration points simultaneously to obtain more reasonable measurements of instantaneous crack length.The J resistance curve derived from the modified separation parameter S_Pb method is higher than that obtained by the normalization method within a limit crack extension;however,the two curves agree very well within a larger crack extension.The value of conditional crack initiation J-integral J_Q from the normalization method is more conservative than that estimated by the modified separation parameter S_Pb method.