The isothermal section of the phase diagram of the Gd-Sm-Co ternary system at 773 K was investigated by X-ray powder diffraction （XRD）, differential thermal analysis （DTA）, optical microscopy and scanning electron microscopy （SEM） techniques. The result shows that the isothermal section consists of 12 single-phase regions, 16 two-phase regions and 5 three-phase regions. Five pairs of corresponding compounds of Gd-Co and Sin-Co systems, i.e., Gd2Co17 and Sm2Co^7, Gd2Co7 and Sm2Co7, GdCo3 and SmC03, GdCo： and SmCo：, Gd3Co and Sm3Co form continuous series of solid solutions. The maximum solid solubility of Sm in Gd4Co3 arid Gdl2Co7 were about 7.2 at.% and 47.8 at.% Sm, respectively. The maximum solid solubility of Gd in Sm5Col9 and SmsCo2 were about 4.7 at.% and 7.6 at.% Gd, respectively. The binary compounds Sm9Co4, GdCos and SmCo5 were not observed at 773 K. No ternary compound was found.
