© 2022 Chinese Society of Rare EarthsIn this study, a new Dy–Eu–Tm co-doped cubic phase stabilized bismuth oxide solid electrolyte system was synthesized by using solid-state reaction method in atmospheric conditions. Before conductivity measurements, X-ray diffraction (XRD) profiles of the annealed samples show that created mixtures have heterogeneous phase, but after conductivity measurements, the face-centered cubic (FCC) crystal structure is stabilized for all samples. Also, the increase in total dopant rate causes an increase in full width half maxima (FWHM) of main peak (111) on the XRD pattern while average crystal size decreases with it. On the other hand, the highest conductivity value was obtained for the sample having 15% total dopant rate with 0.62 S/cm at 750 °C, whose dopants are evenly distributed in mole percent. Differential thermal analysis (DTA) results of the samples having 1:1:1 and 1:2:1 dopant content ratios show that endothermic peak occurs on their DTA curve, indicating crystal structure transformation such as phase transition or order–disorder transition. Also, thermo–gravimetric analysis (TGA) depending on temperature was evaluated in terms of mass loss. According to TGA curves, mass loss for both heating and cooling process can be negligible due to the small fluctuations (2%) on their TGA curves.