The photoreflectance (PR) spectroscopy of a delta-doped GaAs film has been measured at 300 K. Results reveal many Franz-Keldysh oscillations (FKOs) above the band-gap energy, which will enable the electric-field strength to be determined from the periods of FKOs. Since the photovoltaic effect cannot be neglected in PR measurements when using light as both the pumping and probing beams, it is generally assumed that the modulation field delta F is much smaller than the built-in field F so that the periods of the FKOs will not be affected by the pumping beam, However, the induced photovoltage can be over 2/3 of Fermi level at low temperatures and cannot be neglected even at room temperature. Hence, the finite value of delta F needs to be taken into consideration. The effect of delta F on the shapes of PR is discussed, and it is shown that the FKOs of PR oscillate at a frequency corresponding to F-delta F/2. (C) 1996 American Institute of Physics.