Yttrium iron garnet ferrite using the chosen stoichiometry of (Y3)(MnxAl0.8‐xFe4.2)O12 with x = 0.1 and different milling powder sizes were prepared through ball milling for various milling times to study the effect of powder size reduction on the resulting microstructural and magnetic properties. Sintered yttrium iron garnet ferrites were characterized by X‐ray diffraction analysis and scanning electron microscopy. The particle size (D50) of as‐milled calcined powder was decreased using ball milling (from 3.682 μm for a 0.5‐hour‐long milling to 1.606 μm for a 2.5‐hour‐long milling). Scanning electron microscopy analyses confirmed that the sintered grain exhibited a crystal size that was increased from initial values (average crystal grain sizes of 3.5 ± 0.1 μm for 0.5 hour of milling) up to 6.2 ± 0.1 μm after 2.5 hour of ball milling and the subsequent sintering process. The same sintered specimen after 2.5 hour of ball milling exhibited an obvious increase in saturation magnetization (4πMs), remanence (Br), and squareness ratio (namely Br/4πMs); it also caused a notable decline in coercivity (Hc) and ferromagnetic resonance line width (∆H), which were attributed to the introduction of a smaller size of calcined powder after milling and subsequently resulted in a larger sintered grain. Furthermore, a sufficient spin‐wave line width (∆Hk) and low insertion loss (|S21|) were obtained for the operation of the microwave device. The aforementioned results are all beneficial to the use of yttrium iron garnet ferrite in microwave applications. A correlation between the calcined powder size after milling, grain crystal size after sintering, and magnetic properties was evident in this study. The strict control of calcined powder size after milling is critical in tailoring suitable magnetic properties for yttrium iron garnet ferrite manufacturing processes.