MgAl2O4 transparent nano-ceramic samples have been successfully prepared by ultrahighpressure- sintering method. The ultrafine MgAl2O4 nano-powder was obtained by high-temperaturecalcination method. The samples were sintered at relatively low temperatures (540, 620 and 700°C) under ultrahigh pressures (3, 3.7 and 5GPa) with a six-pressure-source and hexahedral anvil machine that is customarily used for diamond synthesis. The average grain sizes of the prepared samples range from 50 to 75nm. All of the samples sintered under different conditions are transparent. The sample sintered at 620°C under 3.7GPa has the highest density as well as the highest light transmittance (about 50%) at 1300 ~ 1700 nm. The results showed that the ultrahigh-pressure-sintered samples exhibit unique light transmission properties though their densities are not much higher (<97%), compared to the samples, of which densities are higher (>99.5%), obtained by conventional hot-pressing or hot isotopic pressing sintering methods. The light transmittance, its affecting factors and transparent mechanism of the MgAl2O4 samples have been discussed.