We evaluated the thickness effect on fracture toughness in freestanding Cu films with thicknesses of approximately 100, 500, and 2600 nm, on the basis of crack tip opening displacement (CTOD) concept by means of in situ (field emission scanning electron microscopy) FESEM and (transmission electron microscopy) TEM fracture toughness testing. During testing, an acute pre-crack tip blunted gradually and necking deformation in the out-of-plane direction occurred in the region ahead of the crack tip in all the specimens. A crack then initiated from the pre-crack tip and propagated along the necking region resulting in chisel point fracture. Since the small scale yielding condition was not satisfied in the 500 and 2600 nm specimens, the critical CTOD, , at the onset of crack extension was directly evaluated from the in situ images. The results indicate a clear thickness effect on (i.e. decreases as the thickness decreases). Normalized critical CTOD, defined as (where is the thickness), of 100 and 500 nm specimens are similar, . This suggests the presence of a fracture criterion, , in the submicron scale regardless of the film thickness. On the other hand, of the 2600 nm specimen was roughly half of those of the 100 and 500 nm specimens.