Terbium gallium garnet (TGG) ceramics were successfully fabricated by air sintering at 1500 ℃ for 3 h combined with HIP post-treating at 1550 ℃ for 3 h under 150 MPa argon gas, where the TGG powders were synthesized by the co-precipitation method employing ammonium hydrogen carbonate (AHC) as precipitant. The influences of ammonium hydrogen carbonate to metal ions molar ratio (R value) on phase composition and morphology of the resultant powders as well as optical transmittance and Verdet constant of the TGG ceramics were investigated systematically. The precursors with R=3.6, 4.0 and 4.4 calcined at 1100 ℃ form pure TGG phase, whereas the precursor with R=3.2 treated at the same temperature yields the mixed phases of TGG and Ga₂O₃. The TGG powder with R=4.0 shows the best dispersity and homogeneity, giving rise to ceramic with the best optical quality. On the contrary, the powder with R=4.4 exhibits a strong agglomeration, which is closely related to the morphology of its precursor. High quality TGG transparent ceramics with the transmittance of 80.1% at 1064 nm can be fabricated by the nanopowder with R=4.0, and the Verdet constant of the TGG ceramics at 633 nm is rather close to that of the commercial TGG single crystals (-134 rad·T ^-1·m ^-1).