Stress intensity factors in cracked bodies have been calculated by various calculation methods which always have advantages and disadvantages. Especially, the so called conventional method ; i.e., the stress extrapolation method or the displacement extrapolation method has an important advantage of convenience. But the accuracy of stress intensity factor by the methods is not so high because of error due to high order terms. In this paper, errors of stress intensity factors by the stress extrapolation method and the displacement extrapolation were investigated on a mixture mode type crack model in an infinite plate. And an improved technique called V-line extrapolation method is proposed to decrease the curvature of calculated extrapolation data, therefore higher accuracy can be obtained