A versatile method of measuring electro-optic coefficients (r(ij) : i = 4-6 and j = 1-3) of isometric, uniaxial, and biaxial crystals without crystal coordinate rotation has been developed. It has been confirmed theoretically and experimentally that the change in the retardation of an X-cut Y-propagation LiNbO3 crystal depends on the square of the electric field along the X-direction. The constant stress electro-optic coefficient r(51) of the LiNbO3 crystal is found to be 32.05 +/- 0.01 pm/V at 632.8 nm. (c) 2013 The Japan Society of Applied Physics