A new method of determining photon energy spectra has been developed with the activities induced by various photonuclear reactions. Test calculations by the orthonormal expansion method show that this method is applicable to the determination of the bremsstrahlung spectra above about 5 MeV when the activation rates and the cross-section data are given as error-free. The experiments were performed for iron and tungsten targets bombarded by 15-MeV electrons from a linear accelerator and the bremsstrahlung spectra obtained were in agreement with those of approximate calculation. This activation method has the advantage of being available for measuring the spectrum of a bremsstrahlung burst and being suitable for determining the distribution of the photon energy spectrum in the medium.