A high transmittance/small grain size combination for pure spinel ceramics from commercially available nanopowders without sintering aids can be obtained by SPS sintering. By using a low heating rate ≤10°C/min and a sintering temperature ≤1300°C, a transparent polycrystalline MgAl 2 O 4 spinel was fabricated by SPS with an in-line transmission of 74% and 84% for 550nm (visible) and 2000nm (NIR) wavelengths respectively. A small average grain size of about 250nm was obtained and the pores located at the multiple grain junctions have a mean size of about 20nm. The high in-line transmission is linked not only to the low residual porosity but particularly to the very small size of pores.