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A significant coercivity enhancement of the commercial NdFeB magnets with the magnetic properties of (BH)max = 48.4 MGOe and iHc = 17.5 kOe through grain boundary diffusion (GBD) with low-melting Tb55R20Cu25 alloys is demonstrated. Adopting Tb55R20Cu25 alloys as GBD sources is effective in increasing coercivity to 29.0 kOe for R = None, 23.8 kOe for R = Y, 25.6 kOe for R = La, 28.0 kOe for R = Ce, respectively. Yet, (BH)max is slightly reduced to 46.2-48.2 MGOe. The preferential appearance of Cu at grain boundary and triple junction of the grains, and the core-shell structure occurred due to Tb at grain surface remarkably enhance the coercivity. Interestingly, higher coercivity enhancement per wt% Tb usage (ΔiHc/wt%Tb) of 7.2 kOe/wt% for the magnet with Tb55Ce25Cu25 than 5.9 kOe/wt% for that with Tb75Cu25 has been found due to the magnetic isolation effect caused by the preferential appearance of Ce at grain boundary, though a slight lower coercivity enhancement was found for the samples with R = Y and La. Lower melting point (637 °C) for Tb55Ce20Cu25 than Tb75Cu25 (743 °C) leads to larger diffusion depth of Tb into the magnet and therefore contributes to higher efficiency of coercivity enhancement for the magnet with R=Ce.