In this paper, effects of secondary factors such as stress ratio, load range and mean stress are investigated on fatigue crack growth behavior of structural steel. CT test specimens made by structural steel of SWS 58 which is most frequently used in welded bridge are used for fatigue test of this paper. From this study, it is known that fatigue crack growth behavior of structural steel depends not only on variation range of stress intensity factor but on stress ratio, and the secondary factors affect on the fatigue crack growth behavior of structural steel.