To characterize the stable crack extension, it is commonly used a J-integral R-curve instead of a linear elastic R-curve. This paper presents the formula obtained by modifying Liebowitz's, so that this formula may be applied to the investigation of crack extension force using a J-integral R-curve for the ductile metal sheets. The effects of specimen geometry, size and initial crack length on J_R_0 at the onset of stable crack extension are discussed by using this method for the stainless steel sheets, SUS 430, with a thickness of 1.2mm. The stable crack extension phenomena of the same specimens are analyzed by considering the tearing modulus obtained by this method.