We conducted crack propagation experiments on freestanding copper films with thicknesses ranging from about 800 to 100nm deposited by electron beam evaporation to clarify the size effect on fracture toughness in the nano-scale. It was found that initially, the crack propagated stably under loading, and then the crack propagation rate rapidly increased, resulting in unstable fracture. The fracture toughness K_C was estimated on the basis of the R-curve concept, showing a clear size effect where thinner films have smaller fracture toughness. The fracture surface suggested that the crack underwent large plastic deformation in the thicker 800-nm and 500-nm films, whereas it propagated with highly localized plastic deformation in the thinner 100-nm film. This size effect in fracture toughness will be related to a transition in deformation and fracture morphology near the crack tip.