Spinel-type Zn_1.1Ga_1.8Ge_0.1O₄ is an excellent host material for dielectric and long-persistent luminescent ceramics. It is helpful to expand application when the transparent ceramics is fabricated. In this paper, the pressureless sintering kinetics of Zn_1.1Ga_1.8Ge_0.1O₄ ceramics were studied. The results show that both densification and grain growth are controlled by the grain boundary diffusion of O^2-. The activation energy for densification in the intermediate sintering stage is determined as (426±35) kJ/mol, while the activation energy for grain growth in the final sintering stage is calculated as (439±14) kJ/mol. When the combination of sintering parameters is optimized to enter into the so-called “sintering window”, the favorable microstructure of the pressureless-sintered body is obtained. After hot isostatic pressed at high temperature, Zn_1.1Ga_1.8Ge_0.1O₄ transparent ceramics with optical transmission ranging from 0.3 μm to 9 μm and in-line transmittance of 81.2% at 2.5 μm are prepared.