AbstractRefractive indices and effective electro‐optic coefficient γc of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γc = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γc of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γc of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO3 (γc = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.