Clamped and unclamped electro-optic coefficients γ 13 and γ 33 of In 3+ /Er 3+ -codoped LiNbO 3 crystals, which were grown by Czochralski method from the melts containing 0.5 mol% Er 2 O 3 while varied In 2 O 3 contents of 0.0, 0.5, 1.0 and 1.5 mol%, were measured by Mach-Zehnder interferometry. The results show that In 3+ /Er 3+ codoping does not cause change of γ 13 and γ 33 , and both γ 13 and γ 33 can be regarded as unchanged in the studied In 3+ concentration range of 0–2.6 mol% (in crystal) within the experimental error of 3%. The small doping effect is desired in light of the electro-optic application of the crystal. A qualitative, comprehensible explanation for the small effect is given on the basis of the EO coefficient model of LiNbO 3 and doping effect on the defect structure of LiNbO 3 .