Fractional release of cesium from coated UO2 particles for the high-temperature gas-cooled reactor (HTGR) is investigated. Data are measured in an irradiation test that utilizes three kinds of coated particles of artificially failed particles; they are simulating through-coatings failed particles, as-manufactured SiC-failed particles, and intact particles. Through the comparison of measured and calculated fractional releases, a model is revised to accurately evaluate cesium fractional release. The fractional releases of cesium from fuel compacts, which are manufactured by the same method as the high-temperature engineering test reactor fuel and are irradiated in the sweep gas capsule, are also evaluated. The result shows that the revised model can accurately predict measured fractional releases.