Scale effects in the high temperature gas pressure forming of electrodeposited fine-grained copper thin sheets were investigated by a series of tests at various forming temperatures and die apertures. The average as-deposited copper grain size was 5 μm. The geometrical parameters of the bugling die system and the thickness of copper sheet varied in proportion. Different radius hemisphere parts from 0.5mm to 5mm were obtained at a strain rate of 5.0×10−4 s−1, which was controlled by pressure forces curves determined in terms of a finite element method (FEM) based on constitutive equation proposed by Backoften in 1964. The experimental relative bulging height (RBH) values were measured, and compared with that predicted by the same finite element method (FEM). It was found that the experimental values of large scale parts approach to simulated values, whereas the experimental values of small scale parts were quite different from simulated values. In order to explain these phenomena, a grain-rotation-weakened mechanism was proposed.