The surface symmetry of hemicyanine monolayer and merocyanine monolayer is studied by surface second‐harmonic generation. While the monolayer consisting of dye monomer has C∞v, symmetry with the rotation axis along the surface normal, the monolayer containing J‐aggregated merocyanine has C2vsymmetry whose rotation axis lies in the surface plane. This anisotropy in the surface plane is diminished by heating due to the dissociation of the J‐aggregates.