5.0at% and 10.0at% Mn-doped MgAl2O4 transparent ceramics have been prepared through vacuum sintering (1750°C /2h) and subsequent hot isostatic pressing (HIP) (1650°C /2h). The effect of microstructure on the light transmittance of the sintered MgAl2O4 is discussed. Results showed that the grains became dense and grain boundary became clear and most of pores inside and between grains were removed, so the transmittance of two samples increased. The optical transmittance of the 5.0at% specimens was increased by one times in the visible light wavelength region. HIP treatment has a great effect on improving the transparency of samples with low doping concentrations.