A new conversion process combined with a three-dimensional (3D) printing technique has been developed for application in the fabrication of a ceramic core. The new process allows the provision of a sufficient amount of inorganic binder on the particle surface and at the interface between particles. Two types of polyvinyl alcohol (PVA) with a similar molecular structure, but with a large difference in decomposition temperature were used as an organic binder. A green body was prepared with two kinds of PVA using a 3D printer, which was heat-treated at 250 °C to evaporate the PVA with the lower decomposition point. The heat-treated core samples were dipped into the inorganic precursor, and then dried and heat-treated at 1000 °C for the organic–inorganic conversion process. In the new conversion process, the formability of the core sample was controlled through the remaining organic compounds and the inorganic binder penetrating between particles and/or coated on the particle surface, and the firing strength was attributed to the glass phase generated by the inorganic precursor. The firing strength of the core prepared through the new process was improved because of the increase in the inorganic precursor that infiltrated the spaces/sites of the decomposed PVA, and it could be readily applied to the production of a core using 3D printing techniques without further shrinkage in a high-temperature heat treatment.