In this article, the mechanism of resonant enhanced nonlinearity is introduced in rare-earth doped fibers. The nonlinearity data of several rare-earth doped fibers are presented. The nonlinearity of rare-earth doped fibers is much greated than that of common silica fiber. However, the nonlinearity respond speed in rare-earth doped fibers is lower than that in common silica fiber. At the same time, the nonlinearity enhancement is correlative with pump and dopant concentration. The resonant enhanced nonlinearity of Yb∶Er codoped fiber Bragg grating has been investigated by experiment. The transmission spectrum of the FBF with various incident powers of 980 nm laser is recorded. The Bragg wavelength was shifted 0.2 nm with 100 mW incident power. This experiment demonstrates that the nonlinearity of rare-earth doped fiber Bragg grating has high nonlinearity coefficient as high 106 times as common silica fiber gratings. This type FBG has some application potentials in all-optical switch.