In fatigue crack growth test, it is important not only to analyze characteristics of fatigue crack growth but also to determine the threshold stress intensity factor, ${Delta}K_{th}$. which is the threshold value of fatigue crack growth. Linear regression analysis using fatigue test data near the threshold is suggested to determine the ${Delta}K_{th}$ in the standard test method but the ${Delta}K_{th}$ can be affected by a fitting method. And there are some limitations on the linear regression analysis in the case of small number of test data near the threshold. The objective of this study is to investigate differences of the ${Delta}K_{th}$ due to regression analysis method and to evaluate the relative error range of the ${Delta}K_{th}$ in same fatigue crack growth test data.