A new phenomenon that the electroabsorption of an‐p‐nheterodiode is enhanced by the absorbed light is reported. The electroabsorption is essentially based on the Franz‐Keldysh effect, where the light absorption edge of the heterodiode shifts towards a longer‐wavelength region under the applied voltage. It is shown that the optical enhancement of the electroabsorption is caused by the space charges accumulated through trapping of photogenerated free carriers.