The spin damping behavior of the bismuth-substituted yttriumirongarnets(BixY3−xFe5O12) prepared by the coprecipitation process was investigated using the ferromagnetic resonant (FMR) in this study. By analyzing the FMR responses, the spin damping coefficient of the x=0.25–1.25 in polycrystalline Bi:YIG (yttrium aluminum garnet) samples was obtained from the 3 dB linewidth of their FMR spectra. The data demonstrated that the damping coefficient started from 3.21×10−3 for x=0.25 and went to its minimum value of 0.31×10−3 for x=0.75. However, the damping coefficient enhanced to 8.55×10−3 with increasing of the bismuth concentration to x=1.25. The results revealed that the spin damping behavior of the Bi:YIG garnets was strongly dependent on a YFeO3 (111) phase.