A new way to evaluate the diffraction efficiency of the diffractive optical element (DOE), which serves as an aberration corrector in an infrared hybrid optical system, is proposed. The directly measured ensquared energy of the hybrid optical system is obtained through an experimental test. Then, the actual line spread function (LSF) of the system under test is measured, and the point spread function (PSF) is calculated from LSF based on the symmetric property of the system. The PSF is used to calculate the so-called derived ensquared energy. The ratio of the directly measured and the derived ensquared energy, defined as the diffraction efficiency of the DOE, represents the fraction of the energy focused onto the image point over the total input energy on the image plane, because the energy loss is caused by the scattering from the DOE corrector. The hybrid optical system, experimental setup, evaluation procedure, and the final results will be presented.