Abstract: A method for the determination of gadolinium isotope abundance ratio of irradiated gadolinium by thermal ionization mass spectrometry (TIMS) has been established. The natural gadolinium oxide was prepared into a target and placed in the preset channel of high flux engineering test reactor (HFETR) to receive neutron irradiation. The total irradiation time was 91.3 h, the reactor power was 80 MW, and the neutron flux rate of irradiated channel was about 2×10¹⁴ n•cm^-2•s^-1. After irradiation, the radiation and measurement of gadolinium (Gd) of flammable neutron poisons were completed by cutting, transporting, dissolving, sampling and measuring the targets,then the measured values were corrected. The results showed that the analyzing date for the post-irradiation testing of Gd by TIMS was accurate and reliable. The post treatment method of the sample after irradiation was reasonable. The variation of the isotopic abundance of various gadolinium was consistent and closely related to the neutron absorption cross section. This method can be used for isotopic analysis of gadolinium in uranium/gadolinium mixed fuel pellets and unloading element. After the measured data are obtained, it can be used to revise the theoretical calculation in order to obtain better reactor core design scheme.