Congruent lithium niobate (LN) doped with rare-earth elements is a promising host for integrated optic combining the properties straightforwardly associated to the dopant with the good electro-optic (EO) properties. By direct technique based on interferometric optical arrangement at a wavelength of 633 nm and at room temperature, we have experimentally determined the figure of merit F = n 3 r eff , (n is the refractive index and r eff , the effective EO coefficient) and finally calculated the EO coefficients r eff of the third-column of the unclamped EO tensor of three series of singly doped LN with Yb 3+ , Ho 3+ , Tm 3+ and two series of doubly doped Er 3+-Yb 3+. It is found that in all the studied opto-geometric configurations, the unclamped figure of merit and consequently the corresponding EO coefficients are relatively constant in the considered dopant concentration range. As the figure of merit, F qualifies crystals for EO modulation and laser applications, all reported results confirm that the LN singly or doubly doped with rare-earth elements are very promising versatile candidates for several multifunctional nonlinear devices in optoelectronic and laser applications.