Composition effect on electro-optic (EO) properties of a LiNbO₃ (LN) single-crystal has been investigated in a Li₂O-content range of 47.0–49.95 mol%. Some non-congruent LN crystals with different Li₂O-contents were prepared by performing Li-deficient or Li-rich vapour transport equilibration treatments on as-grown congruent LN crystals. Unclamped EO coefficients γ₁₃ and γ₃₃ of these samples were measured by a Mach–Zehnder interferometric method. The measurements show that in the Li-deficient regime both γ13 and γ33 increase by ∼8% as Li₂O-content decreases from the congruent 48.6 mol% to the 47.0 mol% in the Li-deficient regime. The feature is desired for the EO application of the Li-deficient crystal. In the near-stoichiometric regime, both γ₁₃ and γ₃₃ reveal a non-monotonic dependence. As the Li₂O-content increases from the 48.6 mol%, the EO coefficient decreases. Around Li₂O-content 49.5 mol%, a minimum is reached. After that, the EO coefficient recovers slowly. At the stoichiometric composition, it recovers to a value close to that at the congruent point. Comparison shows that different crystal growth methods give rise to different defect structure features and hence different composition effects.