Carbon fiber reinforced plastic, CFRP, plates bonding method is one of the new repair methods for fatigue cracks. In this method, propagation of fatigue crack will be delay because of the suppression of crack opening by crack bridging and stress reduction by composite effect of bonding CFRP plates. In this study, to clarify the effect of bonding CFRP plates on fatigue life extension, bending fatigue tests were carried out. First, fatigue tests on the specimens in as-welded condition were conducted until fatigue crack propagated to 10mm away from the weld toe. Then, fatigue crack was covered by CFRP plates and the specimens were re-tested until fatigue crack penetrated through the base plate. As the results of bending fatigue tests, the fatigue test data of the specimens repaired by bonding CFRP plates showed much longer fatigue life compared with that in as-welded condition, under stress ratio, R =0 and -infinity