The surface potential built-in vacuum deposited phthalocyanine (Pc) film on aluminium electrode is investigated by the electro-modulation method. To avoid the influence of charge injection, a sandwiched cell (Al-Pc-airITO) is fabricated. The modulation signal, which depends on the exteal electric field intensity, is detected.The properties of electro-absorption (EA) and electro-reflectance (ER) signals are analysed. The existence of the surface potential at the metal/organic-material interface induces 1f referenced ER signals. As a result, the surface potential built across the vacuum-deposited Pc film on the aluminium electrode is estimated to be 1.23 V.