A method of estimating the crystal grain size of a nuclear fuel pellet. A plurality types of UO.sub.2 powders are heated at a predetermined temperature raising speed in dry air so as to measure weight change ratios occurring due to the oxidation of each of the UO.sub.2 powders. From a value at which the weight change ratio changes, a temperature at which the composition of the powder from the UO.sub.2+x phase to the U.sub.3 O.sub.7 phase is determined for each type of the powders. Sintered pellets are produced from the plurality types of UO.sub.2 powders in which the arrival temperatures are known. Crystal grain sizes of the plurality types of the sintered pellets produced are measured. According to the U.sub.3 O.sub.7 phase arrival temperature determined and the crystal grain size of the sintered pellet measured, a correlation between the both is recognized. A U.sub.3 O.sub.7 phase arrival temperature of a UO.sub.2 powder of a test sample is determined under the same conditions as described above. According to the phase arrival temperature and the above-mentioned correlation, a crystal grain size of the UO.sub.2 powder of the test sample upon production into a sintered pellet is estimated.The crystal grain size of a UO.sub.2 sintered pellet manufactured from the UO.sub.2 powder can be estimated from the oxidation behavior of the UO.sub.2 powder without actually manufacturing the sintered pellet. Thereby the measurement cost for the crystal grain size can be reduced.