The transparent nanocrystalline MgAl2O4 spinel ceramics were synthesized at lowtemperature and high-pressure conditions with low-cost nano-sized MgAl2O4 powder. The sintering characteristics of transparent nano-ceramics were investigated at 500~700oC under 2 ~ 5 GPa. The optimal sintering condition for preparing transparent nano-ceramics was determined. The microstructure and phase composition of powder, as well as the microstructures, morphologies, optical properties, densities and mechanical properties of synthetic ceramics were investigated. The grain sizes of the synthetic ceramics are less than 100 nm, far smaller than those of usual transparent micron-ceramics, and the average grain size depends on the pressure and temperature. The transmittance at the saturation plateau can near 80%. The relative densities of all samples are less than 99%, however, they are highly transparent. The toughness, derived from energy dissipation, of the transparent nano-ceramics was investigated and was compared with that of transparent micron-ceramic.