The Japan Atomic Energy Agency (JAEA) has started to study and develop zirconium carbide (ZrC)-coated fuel particles for advanced high-temperature gas-cooled reactors. The ZrC-coated particles have been fabricated at JAEA and then heat-treated to investigate the effects of the fuel compact sintering process. Remarkable ZrC crystal grain growth occurred in one batch, but not in another, in spite of both having the same C/Zr ratio and ZrC density. TEM/STEM observation of the specimens before heat treatment clarified that considerably more free carbons or voids, which would be expected to hinder the grain growth of ZrC with heat treatment, were distributed in the ZrC layer in the batch in which the remarkable ZrC crystal grain growth did not occur than in the other batch. Differences between these batches, which could not be detected by estimating the C/Zr ratio or the ZrC density, could be detected with positron annihilation spectroscopy (PAS). This result indicates that there is a possibility that PAS can be applied for quality control for ZrC-coated fuel particles in the future, as preparing specimens for PAS is much easier than for TEM/STEM observation. Moreover, PAS is much more suitable than microstructural observation for quantitative estimation. Characterization of defects detected with PAS would be the next step in this process for the investigation of the feasibility of the application of PAS to the quality control of ZrC-coated fuel particles.