YAG nanopowders were synthesized by a co-precipitation method using ammonium hydrocarbonate and ammonia water as the precipitants respectively. The influences of precipitants on chemical compositions, phase transformation and sinterability of the prepared powders, and transmittance of the vacuum-sintered YAG ceramics were studied. The sinterability of powders synthesized using ammonium hydrocarbonate as precipitant is better than that with ammonia water. Pure YAG phase can be obtained by calcining the hydrate precursor at 1200°C, while some impurity phases exist when calcining the carbonate precursor at the same temperature. Transparent YAG ceramics were fabricated by vacuum sintering at 1700°C for 5 h using the YAG nanopowders, and their in-line transmittance is about 60% in the visible light range.