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The effect of prestrain on fatigue crack growth characteristic (FCGC) of age-hardened Al 6061-T6 was investigated. Al 6061-T6 specimens were subjected to tension to produce plastic strain. Extruded bars were prepared to investigate FCGC. Specimens with an artificial hole from the bar were used to observe Mode I fatigue crack. On the other hand, coarse-grained plain specimens were used and observed for Mode II fatigue crack. Fatigue tests were performed under a rotating bending load (R=−1). The experimental results showed the Mode I fatigue crack growth rates decelerated a little due to prestrain. However, in the case of Mode II, fatigue crack growth rates of prestrain specimens accelerated about ten times due to prestrain. The Mode II fatigue crack easily propagates on the slip bands, because precipitation particles on the slip bands are sheared by prestrain. The result means the influence of prestrain on Mode II fatigue should be taken into account for damage-tolerant design.