Abstract: The reactors in HTR-PM are going to undergo an intermediate process before they come to equilibrium. In the intermediate process, two kinds of fuel pebbles, which are different from each other only in initial enrichment of fuel, will be put into the reactors. It is required during the operation that the fuel pebbles with lower fuel enrichment ought to be identified, then discharged according to their burnup level. Therefore, this paper focuses on how to distinguish these two types of fuel pebbles by the means of analyzing the activity (or activity ratios) of radionuclides in irradiated fuel pebbles. By comparing output from calculation of reactor physics software and KORIGEN, it’s preliminarily concluded that it is feasible in the view of theory to discriminate pebbles with different fuel enrichments by analyzing radionuclides in fuel pebbles and available indicators including activity of ¹³⁴Cs, the ratio of activity of ¹²⁵Sb to activity of ¹³⁷Cs and the ratio of activity of ¹³⁴Cs to activity square of ¹³⁷Cs