YAG transparent ceramic has been successfully fabricated by the vacuum sintering technique, with MgO as sintering aids. Thermal expansion curves of the green samples, optical transmittance spectrum, and surface microstructure of the prepared YAG ceramics were measured. The effect of MgO content on the microstructure, in-line transmittance of the samples was systematically studied. Transparent ceramics with high optical quality were obtained after vacuum sintered at 1780 ℃ for 6 h. Ceramics with MgO at the level above 0.1 wt.% were easy to densify, however, the second phase and pores appeared then, which did harm for the optical quality of the samples. 0.03 wt.% of MgO content benefited the sintering process most. Uniform grains with about 10μm were gained, with neither inner nor intergranular pores. The in-line transmittace of the ceramic at 1100 nm reached 82%.