Nano-sized MgAl2O4 powders were treated at 500~1000 oC in a vacuum of 10-3Pa for 2 h, and then sintered at 500~900 oC under 1~6 GPa. The powders with and without vacuum-heat-treatment were investigated by means of Transmission Electron Microscope (TEM) and Sedimentation Test. It was found that the vacuum-heat-treatment of nano powder could effectively get rid of the absorbed gas on the surface of nano-particles. Bulk nanostructured transparent ceramics without cracks were sintered easily with the treated nano-sized powder. The success rate of nano-sinterning of bulk transparent ceramics was greatly increased when a suitable vacuum-heat-treatment process was used for the starting nano-sized powder.