In this Letter, ceramic Nd:YAG is charactrizeby electron spin resonance (ESR) measurements. The ESR results indicate that the polycrystalline ceramic Nd:YAG has barely native defects and impurity ions localization defects, compared to an Nd:YAG crystal with the same Nd doping concentration, due to its density structure by sintering in a vacuum pure raw material and additives during the fabrication. It may conclude that the high quality ceramic Nd:YAG may have greater ability on optical characteristic, mechanical performance, and laser damage than that of the crystals, which is a promising candidate to use on laser diode-pumped solid-state lasers.