Although the metal crown is aesthetically improved when porcelain is fused to it, a large number of different factors often cause the porcelain to break. Here we report a method to prevent this from happening. As a hardening treatment for the porcelain surface, the Na-ions in the porcelain, which have a short radius, were replaced by K-ions having a long radius to strength the surface against compressive forces.1. The bending test showed that the strength of the porcelain had increased from 20 to 38%. 2. The hardness test indicated that the strength had increased from 23 to 24%. 3. The breaking test showed that although the strength of the porcelain had not increased to any great extent, the strength of the veneer shell had increase by more than 30%. 4. The EPMA measurement showed that ion replacement had penetrated to as much as 50 to 80 microns. 5. Clinical tests indicate that even after 19 months, the surface of the porcelain remains the same and that the condition is quite stable.