The cobalt doped magnesium aluminum spinel is an efficient material for the passive Q-switching solid state lasers operating at the near infrared region. In order to prepare Co:MgAl₂O₄ transparent ceramics with high absorption cross section and high in-line transmittance, the cobalt doped magnesium aluminum spinel nanopowders with pure phase were synthesized via the co-precipitation method. And after vacuum sintering and hot isostatic pressing (HIP), highly transparent 0.05at% Co:MgAl₂O₄ ceramics were obtained. The influences of pre-sintering temperature on morphology and optical property of the Co:MgAl₂O₄ ceramics were studied. With the pre-sintering temperature increasing from 1500-1650 ℃, the relative density increased from 95.2% to 98.5%, while the relative density decreased to 97.7% with the pre-sintering temperature increasing to 1700 ℃. Because of the relative low density, the Co:MgAl₂O₄ ceramics were all opaque after vacuum pre-sintering. In the sintering temperature range of 1500-1700 ℃, the average grain size increased from 2.3 to 11.3 μm. After HIP post-treatment, the Co:MgAl₂O₄ ceramics pre-sintered from 1550 to 1700 ℃ were all transparent, the Co:MgAl₂O₄ ceramics pre-sintered at 1650 ℃ for 5 h and HIP post-treated at 1800 ℃ for 3 h showed the best optical quality, which were 81.4% at 400 nm and 85.9% at 900 nm, and the average grain size was 16.8 μm. The broad absorption bands in the wavelength range of 500-700 nm and 1200-1600 nm indicated that Co ²⁺ had incorporated into the spinel lattice. Moreover, the ground state absorption cross section of the best specimen is calculated with a value of 6.18×10^-19 cm² at 1540 nm, meaning that it’s a promising candidate for passive Q-switching in solid-state lasers.