Abstract: The magnetic properties and thermal stability of sintered Nd-Fe-B magnets were studied by grain boundary diffusing Dy₇₀Al₁₀Ga₂₀ alloy. The changes of magnetic properties and microstructure of the magnets were measured by NIM-500 C and SEM. The results show that after Dy₇₀Al₁₀Ga₂₀ alloy receiving thermal diffusion treatment, its coercivity is enhanced significantly from 1 080.968 kA/m to 1 671.600 kA/m, a striking increment of 55 %. And there is only a slight reduction in remanence. Dy, Al and Ga elements diffuse at the grain boundary, which effectively insulates magnetic exchange so that the coercivity of the alloy is well improved. It is obvious that after Dy₇₀Al₁₀Ga₂₀ alloy is diffused, in the epitaxial layer of grains is a gray shell covering the main phase grains via SEM, which plays an active role in magnetic exchange to isolate the grains. The peak of the main phase is generally shifted to the right via XRD observation, given the fact that the heavy rare earth element Dy enters the epitaxial layer of grains and forms the (Nd, Dy) core shell structure. The atomic radius of Dy is smaller than that of Nd, causing the peak to move to the right.