The major drawbacks of Nd-Fe-B magnets are relatively low Curie temperature and poor thermal stability. Ribbons with the near stoichiometric 2:14:1 composition of Nd10.8Dy0.75Tb0.75Fe79.7?xCoxZr0.8Nb0.8Cu0.4B6.0 (x=0, 3, 6, 9, 12, 15) were prepared by rapid quenching and subsequent heat treatment. The effect of Co element on the magnetic properties, thermal stability, and microstructure of the ribbons was systematically studied by vibrating sample magnetometer (VSM), thermal magnetic analysis, atomic force microscopy (AFM), and transmission electron microscopy (TEM). It was found that Co substitution was significantly effective in improving the magnetic properties and the thermal stability of nanocrystalline ribbons. Although the intrinsic coercivity decreased from 1308.7 kA/m for x=0 to 817.4 kA/m for x=15, the remanence polarization and maximum energy product increased from 0.839 T and 116.5 kJ/m3 for the Co-free samples to 1.041 T and 155.1 kJ/m3 for the 12at% Co-substituted samples, respectively. About 10 K increase in Curie temperature was observed for the 2:14:1 phase with 1at% Co substitution. The absolute values of temperature coefficients of induction and coercivity were significantly decreased with Co substitution, which may be attractive for high operational temperature applications. The microstructure of nanocrystalline ribbons was slightly refined with Co substitution. The major drawbacks of Nd-Fe-B magnets are relatively low Curie temperature and poor thermal stability. Ribbons with the near stoichiometric 2: 14: 1 composition of Nd10.8Dy0.75Tb0.75Fe79.7? XCoxZr0.8Nb0.8Cu0.4B6. The effect of Co element on the magnetic properties, thermal stability, and microstructure of the ribbons was systematically studied by a vibrating sample (0, 3, 6, 9, 12, 15) were prepared by rapid quenching and subsequent heat treatment It was found that Co substitution was significantly effective in improving the magnetic properties and the thermal stability of nanocrystalline ribbons. Although the intrinsic coercivity (VSM), thermal magnetic analysis, atomic force microscopy (AFM), and transmission electron microscopy decreased from 1308.7 kA / m for x = 0 to 817.4 kA / m for x = 15, the remanence polarization and maximum energy product increased from 0.839 T and 116.5 kJ / m3 for the Co-free samples to 1.041 T and 155.1 kJ / m3 for the 12at% Co-substituted samples , respectively. About 10 K increase in Curie temperature was observed for the 2: 14: 1 phase with 1 at% Co substitution. The absolute values ​​of temperature coefficients of induction and coercivity were significantly decreased with Co substitution, which may be attractive for high operational temperature applications. The microstructure of nanocrystalline ribbons was slightly refined with Co substitution.