The irradiation behavior of graphite is essential for its applications in the nuclear industry. However, the behavioral differences of graphite remain obscure because of the very limited comprehension of its microstructural differences. One typical structure, the quinoline-insoluble (QI) particle, was investigated using IG-110 and NBG-18 graphite. After irradiation, the QI particles on the polished surface were proven to become hillocks, which were easily identifiable via scanning electron microscopy (SEM). Thus, a method that combined ion irradiation and SEM charac-terization was proposed to study the distribution and con-centration of QI particles in graphite. During irradiation, the QI particles were found to evolve into densified spheres, which were weakly bonded with the surrounding graphite structures, thereby indicating that the densification of QI particles did not evidently contribute to graphite dimensional shrinkage. A much higher concentration of QI particles in NBG-18 than IG-110, which was suggested to be responsible for the smaller maximum dimensional shrinkage of former over the latter during irradiation, was characterized.