Contribution of diffused Er3+ ions to refractive index of LiNbO3 crystal and relation of Li out-diffusion to Er3+-diffusion condition have been studied. Three X-cut and three Z-cut congruent LiNbO3 plates locally coated with 10, 15, or 20 nm thick Er metal films were annealed at 1100 degrees C or 1130 degrees C for 60 or 100 h under different atmospheres of surrounding air, wet O-2 or wet Ar. After the anneal, the ordinary and extraordinary refractive indices at Er3+-doped and Er-free parts of crystal surface were characterized at the 633, 1311, and 1553 nm wavelengths by measurement of critical angle. The difference of surface index values at the Er3+-doped and Er-free parts enables to identify the Er3+-doping effect on substrate index. From the known birefringence, the surface Li2O contents at the Er3+-doped and Er-free parts were evaluated. A comparison with the Li2O content data before the anneal treatment allows to obtain the anneal-induced Li2O loss and hence identify the anneal condition effect on the Li out-diffusion. The results show that the contribution of Er3+-doping to the ordinary index is within the measurement error of 1 x 10(-3) and that to the extraordinary index is within +/- 1.7 x 10(-3), which is just slightly out of the measurement error. The diffusion temperature is while the diffusion environment is not the key factor responsible for the Li2O out-diffusion.