Abstract: Neutron irradiation of the nuclear graphites used in high-temperature gas-cooled reactor (HTGR) results in initial bulk shrinkage at low neutron fluence followed by expansion at higher neutron fluence, and it reaches to the performance life when it expands to initial dimension. In the process of dimensional change, the structure and number of pore in nuclear graphite also change obviously. When the neutron fluence arrives at performance life, the property of nuclear graphite decreases quickly with the number of pore increasing. After He⁺，C⁺，Xe⁺ ions irradiation, 200 keV 10¹⁴cm^-2Xe⁺ ions irradiation is established to evaluate the irradiation property of graphite due to changes of pore in nuclear graphite.