Zirconium doped lithium niobate is a promising candidate as a substrate for nonlinear optical applications, since it does not suffer from the so-called “optical damage.” In order to optimize this aspect, the proper Zr concentration has be used, hence the precise determination of the so-called “threshold concentration,” i.e., the concentration above which the photorefractive effect is markedly reduced, is of great importance. In this work, we prepared by Czochralski growth a series of Zr-doped lithium niobate crystals with various Zr content and studied them using structural (high-resolution x-ray diffraction) and optical (birefringence) measurements as a function of the dopant content in the melt. Both the approaches pointed out a marked change in the crystal characteristics for a Zr concentration between 1.5 and 2 mol %, a value which is identified as the threshold concentration.