DC electric field induced molecular reorientation in hemicyanine Langmuir-Blodgett multilayers has been investigated by rotation-angle second harmonic generation SHG. technique. The hemicyanine molecules could align their chromophore axes towards the field direction along the substrate normal, leading to reduction of the dipping-induced in-plane anisotropy and enhancement of the macroscopic optical nonlinearity upon poling at room temperature. With increasing poling field, the SHG intensity increased quadratically and reached an enhancement of an order of magnitude at a field of 1.33=106 Vrm.