Although the previous researches evaluated the fatigue behavior of Al/GFRP laminates using the traditional fracture mechanism, their researches were not sufficient to do it : the damage zone of Al/GFRP laminates was occurred at the delamination zone instead of the crack-metallic damages. Thus, previous researches were not applicable to the fatigue behavior of Al/GFRP laminates. The major purpose of this study was to evaluate delamination behavior using the relationship between crack length (a) and delamination width (b) in Al/GFRP laminate. The details of investigation were as follows: 1) Relationship between the crack length (a) and the delamination width (b), 2) Relationship between the delamination aspect ratio (b/ a) and the delamination area rate ((AD) N/ (AD) All), 3) The effect of delamination aspect ratio (b/ a) on the delamination shape factor (fs) and the delamination growth rate (dAD/ da). As results, it was known that the delamination aspect ratio (b/ a) was decreased and the delamination area rate ((AD) N/ (AD) All) was increased as the normalized crack size (a/ W) was increased. And, the delamination shape factors (fs) of the ellipse-II (fs₃) was greater than of the ellipse- I (fs₂) but that of the triangle (fs₁) was less than of the ellipse- I (fs₂).