The electroreflectance (ER) spectra of Schottky‐barrier Ag/GaAs have been measured at various dc bias voltages (Vbias). It is known that the surface electric field can be deduced from the period of the Franz–Keldysh oscillations (FKO) of ER. Therefore, the built‐in voltage (Vbi) and the carrier concentration in the depletion region can be calculated by the assumption of a constant charge distribution in the depletion region. In this article, we show thatVbican also be determined from the amplitude of the FKO as a function ofVbiasin the forward biased region. TheVbidetermined by the above two different methods have been verified to be in good agreement.