The optimal quasi‐static‐electric bias for the diffraction efficiency of degenerate four‐wave mixing process in a homeotropically aligned nematic liquid‐crystal film is studied. The nonlinearity of this process is characterized by the behavior of field‐induced molecular reorientation. In our theoretical calculation, it is pointed out that the determination of optimal field is crucially influenced by the twist deformation in the reorientational grating and is also subjected to the competing influences of splay and of bend elastic torques on the molecular reorientation. In experiments, the careful measurement of diffraction efficiency is carried out automatically by using a microcomputer to investigate the optimal bias for this nonlinear effect in nematicN‐(4‐methoxybenzylidene)‐4‐butylaniline films. It is unambiguously shown both by theoretical and experimental results that the optimal bias is decreasing with the increasing of grating period and optical intensity.