In this study, we measured the mechanical strength and fracture toughness of feldspathic dental porcelains with different water contents. The mechanical strength of the dental porcelain was measured using a three-point bending test; the fracture toughness of the dental porcelain was measured by a Vickers indentation method. An increased water content was found to decrease both the strength and toughness. This is probably due to easier crack formation or perhaps to a slightly higher crystallinity of the leucite material. We expected and found that it was necessary to study this phenomenon to overcome the failure of feldspathic dental porcelains during refiring after a temporary cementation in the mouth.