At present, a strong limitation to the applications of LN crystals comes from the fact that, under illumination with visible or near-infrared light, there are semi-permanent changes in the index of refraction of the crystal, due to the photorefractive effect (or " optical damage "), causing beam distortion and dramatically decreasing the device efficiency. This drawbacks and limitation exist unless some strategy to reduce the photorefractive effect is implemented [1]. The resistance to optical damage can be increased considerably by changing the LN crystal composition from congruent to stoichiometric [2,3] and/or by adding to LN an appropriate non-photorefractive dopant. Recently, different tetravalent impurity ions (Hf 4+ , Zr 4+ , Sn 4+) were proposed as new non-photorefractive impurity ions [4,5]. Complex investigations of Hf as well as Zr doped congruent composition LN crystals including photorefractive, non-linear optical, compositional, electro-optical [6-9] properties have been shown that tetravalent impurity ions with a threshold concentration of around 2-3mol% can be considered as good candidates for reduction of the photorefractive effect in LN crystals with preserving of its main characteristics. In this work the Zr doped LN crystals are investigated in order to have a clear idea of the threshold concentration of the impurity ion required for the reduction of the photorefractive effect as well as sites occupied by different concentration Zr ions in the structure of LN. Raman measurements were investigated on Zr doped Lithium Niobate crystals with different concentrations. Spectra were treated by fitting procedure and Principal Component Analysis, which both provide results consistent with each other (figure 1). The concentration dependence of the frequency of main low-frequency optical phonons give an insight of site incorporation of Zr ions in the host lattice. The threshold concentration of about 2% is evidenced, confirming that the interest of Zr doping as an alternative to Mg doping for the reduction of the optical damage in Lithium Niobate. These results allow to explain the observed kink that we have found in the EO behavior as function of [Zr] in LN:Zr doped crystals [9]. Figure 1-The frequency of the mode E[TO 1 ] and the scores of PC2 as a function on the concentration of Zr in LN [1] T. R. Volk and M. Woehlecke.