Defect-free three-dimensional (3D)-printed ceramics, such as alumina, zirconia, and titania, have been successfully fabricated by controlling the initial defect formation using a non-reactive diluent, namely, polypropylene glycol (PPG), as an additive in the photocurable resin. By crosslinking with uncured monomers in 3D-printed ceramic green bodies, PPG prevented them from evaporating early, which often results in defect formation at low temperatures (<325 °C). PPG also decomposed before the main resin components, thereby forming interconnected pores, which consequently enabled gases from resin decomposition to escape throughout the green body. The effects of PPG addition on defect formation were systematically studied during the debinding process of an alumina slurry. In addition, defect formation was analyzed through thermogravimetric analysis, computed tomography 3D scanning, Fourier transform infrared spectroscopy, and differential scanning calorimetry. When the 3D-printed, crack-free alumina was sintered at 1650 °C, a 99.2% relative density was achieved. PPG was also applied to zirconia and titania to verify the versatility of PPG in various types of ceramic stereolithography-printed materials. The results suggested that PPG has excellent potential for fabricating various complex and large-volume ceramics without defects through stereolithography.