Controlling residual amount of defects in transparent ceramics is a major challenge for laser applications. This study was focused on microstructural evolution of Nd:YAG ceramics during their reactive solid-state sintering which was correlated to their optical transmittance. From microstructural observations, the microstructural maps and grain size-density and grain size-pore size sintering trajectories of Nd:YAG ceramics were established as a function of silica content. For densities higher than 99.7%, the occurrence of intragranular porosity was correlated to a critical pore radius of 0.16 μm. Silica appears to favor the formation of intragranular porosity which was attributed to the increasing of the grain growth rate compared with the densification one. An analytical model was established by coupling the analytical laws derived from sintering trajectories and the classical theory of light diffusion, allowing to correlate the microstructural features of transparent Nd:YAG ceramics to their optical properties.