Abstract: The effect of grain boundary diffusion Dy₆₀Co₃₅Ga₅ alloys on the magnetic properties and thermal stability of sintered NdFeB magnets was investigated. As diffusion temperature rose, the coercivity (H_cj) of the magnets first increased and then decreased. After the magnets were under diffusion treatment at 890 ℃ for 3 h and were annealed subsequently at 480 ℃ for 5 h, the coercivity reached maxima, increasing from 1 209 kA/m to 1 624 kA/m. At the same time, the remanence of the diffused magnets decreased slightly, from 1.38 T to 1.32 T. When magnetic properties of the original magnet and the grain boundary diffusion Dy₆₀Co₃₅Ga₅ alloy one were tested at high temperatures respectively, the two magnets both showed decrease in coercivity. But the coercivity of the diffused magnet was significantly higher than that of the original one. After the original magnet and the diffused one were exposed to different temperatures for 2 h, magnets′ irreversible flux loss stood 63 % and 45 %, respectively. According to the differential scanning calorimeter (DSC) results, the Curie temperature (T_c) of the diffused magnet was significantly higher than that of the original one, which indicated that the thermal stability of the diffused magnet has been greatly improved. Besides, the X-ray diffraction (XRD) pattern of the diffused magnet showed that its diffraction peaks of RE₂Fe₁₄B phase shifted to the right, compared to the original one. This suggested that a small number of the Dy atoms and Co atoms have entered the main phase grain.