All coefficients of the unclamped and clamped electro-optic tensor of zirconium-doped congruent LiNbO3 single crystals are determined as function of the dopant concentration at room temperature. With a distribution coefficient of zirconium closer to one in the considered range, the dopant concentration is in the range up to 2.5 mol% of ZrO2. The electro-optic coefficients are measured by direct techniques based on interferometric and Sénarmont optical arrangements at the wavelength of 633 nm. It is found that all the unclamped and clamped effective electro-optic coefficients are relatively constant, except for the sample grown with 2 mol% of zirconium. The electro-optic behavior of LN:Zr as function of the dopant concentration was confirmed by dielectric characterizations. The concentration equal 2 mol% of ZrO2 can be considered to a threshold concentration for various physical and optical properties.