Molecular dynamics (MD) simulations are performed to investigate the effects of stress on generalized stacking fault (GSF) energy of three fcc metals (Cu, Al, and Ni). The simulation model is deformed by uniaxial tension or compression in each of , [11-2], and [1-10] directions, respectively, before shifting the lattice to calculate the GSF curve. Simulation results show that the values of unstable stacking fault energy (γ
usf), stable stacking fault energy (γ sf), and unstable twin fault energy (γ utf) of the three elements can change with the preloaded tensile or compressive stress in different directions. The ratio of γ sf/γ usf, which is related to the energy barrier for full dislocation nucleation, and the ratio of γ utf/γ usf, which is related to the energy barrier for twinning formation are plotted each as a function of the preloading stress. The results of this study reveal that the stress state can change the energy barrier of defect nucleation in the crystal lattice, and thereby can play an important role in the deformation mechanism of nanocrystalline material.