The effect of heat treatment on the ferromagnetic resonance of lutecium iron garnet (Lu3Fe5O12 or Lu I G) and manganese ferrite single crystals is investigated. The heat treatments are 1 hour at 700℃ in air and 10 hours at 700℃ in oxygen, used separately. It is found that these treatments have no significant influence on the values of K1/Ms and the g-factor, but the anisotropy of resonance line-width AH decreases appreciably. Although the effect of heat treatment on the △H of Lu IG is small, the AH of manganese ferrite increases several times after each treatment. The causes of the variation of △H with heat treatment are discussed. Meanwhile, X-ray diffraction patterns show that some α-Fe2O3 precipitates in the manganese ferrite sample after it is subjected to heat treatment.