This paper describes the measurements of optical properties and linear electro-optic effect in ferroelectric single crystal (KxNa1-x)0.4(SryBa1-y)0.8 Nb2O6. This crystal exhibits a large birefringence, and has a transparency range from 4000 ? to 5.6μm. The low halfwave voltage of (KxNa1-x)0.4 (Sry,Ba1-y)0.8 Nb2O6 gives a high value n03·rc of 730×10-12m/V, which is the figure of merit for EO modulator materials. The threshold energy of 600MW/cm2 (4pps) for laser damage makes (KxNa1-x)0.4 (SryBa1-y)0.8Nb2O6 attractive for large power laser modulation.