Abstract: The microstructure and magnetic properties of 38UH, 42SH and N50 magnets with and without Dy diffusion treatment were contrastively investigated. The intrinsic coercivity of the magnets by Dy diffusion increases over 400 kA·m^-1 correspondingly but the remanence shows little decrease. The maximum energy product enhances with the increases in intrinsic coercivity and square degree. The evident enhancement of the intrinsic coercivity of the magnets results from the increases in magnetic anisotropy and nucleation field of the extensive layer of Nd₂Fe₁₄B grains. Based on the Fick's first law, the depth, mass concentration and mass fraction of Dy element with process time at a certain temperature can be approximately calculated.