It is crucial to study the effect of radiation on the fiber amplifier devices. In the present paper, the Erbium-ytterbium co-doped fiber amplifier (EYDFA) has been irradiated by a neutron beam of different doses for various exposure times from an Am-241/Be-9 neutron source. The gain and noise figure of the EYDFA have been calculated theoretically and recorded after and before the irradiation to test its performance under the effect of irradiation. In order to show the enhancement in the gain of the fiber amplifier devices, a comparison between the gain of the irradiated EYDFA and Erbium doped Fiber amplifier (EDFA) has been carried out. The calculated results by the proposed model are in good agreement with the experimental ones. It indicates that the gain of EYDFA deteriorates after being irradiated by a neutron dose. Moreover, the gain of irradiated EYDFA has been reduced to 13.8 dB at a dose of 720 Gy.