The fatigue crack growth of 2124-T851 aluminum alloy was investigated by Electron Backscatter Diffraction (EBSD) analysis. The result shows that the mode of fatigue crack growth is mainly transcrystalline, and the preferential orientation changes with the difference of grain orientation. When the crack extends to the grain boundary, the deflection of the crack against the normal extensive path will take place as a result of the misorientation of adjoining grains, but the deflection of the crack in the grain inner is mainly caused by the coarse second phase particles which are poor in coordination of deformability under cyclic stress. The phenomenon of bifurcation during the process of the crack propagation is relative with the specific crystallographic direction, which is caused by the simultaneous movement of numerous equivalent {111}<110> slip system on crack-tip.