Optical second harmonic generation (SHG) measurement has been employed for studying electric field distribution in organic double-layer electroluminescence diode. Taking advantage of the material dependence on the SHG, we measured the electric field induced SHG (EFISHG) from IZO/alpha-NPD/Alq(3)/Al diode. The generated SH signal wavelength was governed by the active organic layers, and two peaks were generated at 940 nm from the alpha-NPD and at 1050 nm from the Alq(3) layers. The SHG intensity was dependent on applied d.c. voltages, but the voltage dependence was totally opposite between the two peaks. Interface carrier charging caused by the Maxwell-Wagner effect accounted for the results. We concluded that the EFISHG provides an effective way to probe the electric field distribution in double-layer electroluminescence diodes. (C) 2011 Elsevier B. V. All rights reserved.