A novel aluminum magnesium oxynitride transparent ceramic with the chemical formula Mg0.27Al2.58O3.73N0.27 was firstly prepared by pressureless sintering of fine single-phase powders at 1875 degrees C for 24 h in nitrogen atmosphere. The ceramic was fully dense with the average grain size of 57.5 mu m. The sample showed excellent in-line transmission from the visible to middle-infrared wavelengths with the maximum transmittance of 84%, which could be attributed to rare pores in the sintering body. The material also exhibited good mechanical properties of Vickers hardness (13.39 +/- 0.18 GPa), fracture toughness (2.46 +/- 0.3 MPa/m(1/2)), and flexural strength (274 +/- 6 MPa).