In this paper, the size effect of thickness on fracture toughness in the fracture behavior of a kind of copper foils was studied experimentally and analytically. Copper foil specimens with thicknesses ranging from 0.02mm to 1mm were adopted. The fracture toughness defined as the value of the critical J integral at cracking initiation was shown to depend on foils’ thickness within a certain range of thickness. For elucidating the relationship between fracture toughness and thickness, microscopic experimental analysis was carried out. Metalloscope results also showed that microscopic structure of different specimens with different thicknesses is also a factor affecting the fracture toughness. In order to explore the effect of pure thickness on fracture, specimens with the same microscopic structure were also investigated.