The translucent Mg-alpha-sialon ceramics have been prepared by spark plasma sintering (SPS) alpha-Si3N4 powder with AIN and MgO as the additives at 1850 degrees C for 5 min. The sample possesses a uniform, dense microstructure under the rapid densification of SPS process. The translucent Mg-alpha-sialon ceramics achieve the maximum transmittance of 66.4% for the sample of 0.5 mm in thickness in the medium infrared region, which could be attributed to the equiaxed microstructure and few glassy phase confirmed by the observation of transmission electron microscopy. The material also exhibits good mechanical properties of high hardness (21.4 +/- 0.3 GPa) and fracture toughness (6.1 +/- 0.1 MPa(.)m(1/2)).