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To improve the poor thermal stability of NdFeB magnets, we add Dy, Co, Al, and Nb into the magnets with nominal composition (Nd0.9Dy0.1)17FebCo5Al1.7B7Nbx (x=0.3–1.5 at. %). We have measured their Hcj at room temperature. The data of irreversible loss (IL) of open‐circuit flux density at different temperatures are also obtained using cylinder samples with L/D=0.7 (P=2). We have also compared the IL of the same sample with different P. The results show that Hcj increases linearly with Nb content in average 4.8 kOe/Nb at. %. The higher the Hcj and/or the longer the sample is, the lower the absolute value of IL is. The IL of sample with x=1.5 at. % and L/D=0.7 is less than 5% even at 150 °C. Nb effectively achieves the thermal stability of the magnets. The SEM has been used to observe the microstructure of the surface and cross section of a typical sample.