Measuements have been taken on the ferromagnetic resonance properties of narrow linewidth single crystal aluminum‐substituted lithium ferrite at room temperature and in the frequency range from 4.0 to 9.0 GHz. The composition of the starting mixture was as follows: Li2CO3 29.35%; Fe2O3 55.33%; and Al2O3 15.32%. The saturation magnetization of the samples was 2640 gauss at room temperature. The resonant linewidth measured 2.5 oersteds at 9.0 GHz and decreases to 1.0 oersteds at 4.0 GHz. The high power data obtained show the change in the microwave magnetic susceptibility as a function of incident power. From the room temperature threshold field (hc) an intrinsic spin wave linewidth (ΔHk) of 0.17 oersteds was calculated. This narrow linewidth, coupled with the relatively high saturation magnetization, indicates that the material can be utilized successfully in filters and low level power limiters.