The asymmetric Maker fringe pattern of third‐harmonic generation in an anisotropic molecular crystal is analyzed to evaluate the third‐order optical susceptibility &khgr;(3). Analysis of a diethyl‐amino‐nitro‐stilbene crystal shows that the molecular axes are not oriented parallel to the crystal surface, but are about 45° relative to the surface. The &khgr;(3)value parallel to the orientation of the molecular axis is estimated to be 1.0×10−11esu, corresponding to about ten times the &khgr;(3)value perpendicular to that axis.