Mode III fatigue crack is usually observed at the root of circumferentially notched round bar. As the crack grows in the depth direction of specimen, the growth rate of crack decreases. In this respect, crack mode transition from mode III to mode I is understood responsible. However, quantitative evaluation on crack mode transition has been hardly done. Therefore, values of stress intensity factor KI under mode III fatigue crack growth tests on a low carbon steel were calculated with using principal stress, as crack mode transition began. Then KI values were compared with crack growth rate curve of mode I. As the result, it was observed that the condition of crack mode transition was given by the crack growth rate curve of mode I.