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Transparent YAG ceramic has become a widespread laser gain medium for solid state lasers for many years,due to its excellent mechanical and optical properties.In order to fabricate transparent YAG ceramic with low scattering loss for laser applications,sintering aids are often applied to further promote densification as well as inhibit rapid grain growth,and tetraethyl orthosilicate (TEOS) has been proved to be an effective sintering aids for YAG ceramic[1].Nevertheless,TEOS has its intrinsic disadvantages,for example,TOES doping easily leads to dramatic grain growth,which is not benefit to the mechanical property of YAG ceramic.Also,TEOS will tremendously deteriorate the conversion efficiencies of specific doping ions like Cr4+ or Yb3+ in YAG ceramics[2].We investigated the microstructure evolution of YAG ceramics sintered at different temperatures in vacuum using CaO and MgO as sintering additives without TEOS doping,as is shown in Fig.1.The average grain size of the CaO doped sample was about 1.7 μm after vacuum sintered at 1780 ℃ held for 4 h,and 4.5 μm for MgO doped YAG ceramics.For CaO doped YAG ceramics,the relative density nearly reached to its theoretical value when the sintering temperature was higher than 1700 ℃,which was 80 ℃ higher than that of MgO doped sample.So it is obvious that although MgO can lower sintering temperature during vacuum sintering to a large extent,but not as efficient as its CaO counterpart in the suppression of grain growth as sintering aid.