The growth- and frequency-dependent microwave studies on ~ 250 nm thick Y₃Fe₅O₁₂ [yttrium iron garnet (YIG)] films deposited on Gd₃Ga₅O₁₂ (111) (gadolinium gallium garnet) substrates by RF magnetron sputtering has been reported. After the deposition, the thin films were ex-situ annealed in air at 700 °C for different time intervals (2, 4, 6, and 10 h) and a pure YIG phase with preferred (111) orientation was formed. Effective saturation magnetization (4π Meff) has been estimated from ferromagnetic resonance (FMR) data using Kittel's equations. A negative uniaxial magnetic anisotropy has been observed which decreased with the increase in annealing time. The FMR linewidth (Δ H) for the 10 h annealed film was found to be nearly half of the one annealed for 2 h. The 10 h annealed film showed a Δ H value which lies between 30 and 50 Oe over a wide frequency range (2-18 GHz). This improvement in the microwave properties as a result of annealing is attributed to the reduction of defects and relaxation of stress.