The isothermal section of the Cu-Co-Nb ternary system at 1198 K has been investigated by means of diffusion triple together with electron probe microanalysis technique. Series of tie lines and tie-triangles have been determined and the isothermal section at 1198 K has been established，and four three-phase fields have been figured out. Namely, (Nb)+(Cu)+Co7Nb6, (Cu)+Co2Nb+Co7Nb6, Co2Nb+Co3Nb+(Cu) and Co3Nb+(Co)+(Cu) existing in the present isothermal section. In present work Co3Nb and Co7Nb2 were disposed as only one phase Co3Nb. It is found that the solubility of Cu in Co3Nb, Co2Nb and Co7Nb6 are only up to 5.70, 5.80 and 4.20 at.% Cu, respectively. The range of homogeneity of Co7Nb6 and Co2Nb is 44.7 to 48.0 at.% and 25.0 to 29.6 at.% Nb, respectively. The Co3Nb is line compounds.