The fatigue crack growth behavior of physically-short cracks(0.2<a<2mm) and long cracks (2<a<25mm) under constant amplitude loading was investigated with SEN type specimens using aluminum 7075-T6 material with stress ratio, R, of 0.05, -1 and 0.5 at room temperature. The crack closure effect on fatigue crack growth was also investigated. It was found that the physically-short cracks grew much faster than long cracks based on applied ${Delta}K$ with $da/dN<1{ imes}10^{-7}m/cycle$. The transition crack lengths where similtude with ${Delta}K$ existed was between 1 and 2mm. The effective stress intensity factor range based on COD measurements gave better correlation between the physically-short and long cracks. Thus it can be considered that the crack closure effect is one of the main factors which causes the differences between these two cracks.