The second‐order nonlinear optical coefficientsd33,d31, andd15of GaN:Mg epitaxial film were studied by the standard Maker fringe of an anisotropic medium. The measuredd33=−(16.5±1.3) pm/V which is 55 times of thed11of quartz. The measured ratios ofd33/d31andd31/d15showed that the crystalline film is close to an ideal wurzite structure. The refractive indices and the dispersive curves ofne,n0were also determined by TM and TE waveguide mode measurements. ©1996 American Institute of Physics.