We investigated the feasibility of directly estimating split-off hole effective mass m so from Franz-Keldysh oscillations using non-destructive and non-invasive photoreflectance spectroscopy. We used an undoped/n-type GaAs epitaxial structure and two GaAs p-i-n diode structures. We observed the phenomenon that Franz-Keldysh oscillations, which originate both the E 0 fundamental transition and from the E 0 + ∆ so transition energy, appear in the photoreflectance spectra at room temperature. Initially, from the electro-optic constant ℏΘ HH of the Franz-Keldysh oscillations originating from the E 0 fundamental transition, we deduced the built-in electric field F in each sample with the use of the relation ℏΘ HH = (e 2 ℏ 2 F 2 /2µ HH ) 1/3 , where the quantity µ HH is the reduced effective mass of the electron and heavy hole. In the next step, we calculated the reduced effective mass µ so of the electron and split-off hole using the relation ℏΘ so = (e 2 ℏ 2 F 2 /2µ so ) 1/3 , where the quantity ℏΘ so is the electro-optic constant of the Franz-Kelysh oscilations from the E 0 + ∆ so transition. Finally, we estimated the split-off hole effective mass m so from the reduced effective mass µ so . The estimated split-off hole effective mass agrees with the reported value obtained experimentally. Thus, we conclude that the split-off hole effective mass is estimatable from analyzing the Franz-Keldysh oscillations in the photoreflectance spectra. We also compare the present hole effective masses to those theoretically known and discuss the appropriateness of the electronic-band calculations.