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We performed photoreflectance (PR) spectroscopy in order to investigate the fundamental band gap of GaAs 1-x Bi x alloys. The temperature dependence of the band gap energy was evaluated by analyzing of the Franz–Keldysh oscillation in the PR spectra. With increasing Bi content, the band gap energy of GaAs 1-x Bi x alloy is reduced, which shows a large optical bowing. The temperature coefficient of the band gap decreases appreciably in alloys with increasing Bi content. For x= 0.026, the temperature coefficient near room temperature is-0.15 meV/K which is 1/3 of the value for GaAs.