Background/Question/Methods Estimation of 14C abundance (14C/total C) in the edible part of crops is necessary to assess the local radiological impact of 14C released from the Rokkasho spent nuclear fuel reprocessing plant in Japan. In the specific activity method generally used to assess radiation dose due to 14C, 14C abundance in crops is assumed to be identical to the annual mean of 14C abundance in air. However, the abundance of heavier carbon isotopes in crops becomes lower than that in air due to isotope fractionation, and it is affected by temporal changes in carbon isotopic composition of atmospheric CO2 because the ratio of carbon assimilated in crops at a given day to the total carbon at the time of harvest (R) is not always constant throughout the crop growth period. In this study, rice plants were exposed to temporal changes in 13C abundance in air throughout the growth period. Results/Conclusions Mean d13C value of air throughout the growth period was –5.2‰ and markedly higher than the measured d13C value of rice grain, –30.9‰. When the d13C value of rice grain was predicted by correcting the mean d13C value of air by the difference in d13C between air and rice grain under natural conditions due to isotope fractionation, the predicted d13C value of rice grain was –23.6‰. When both isotope fractionation and temporal changes in the d13C value of atmospheric CO2 throughout the growth period were taken into account, the predicted d13C value of rice grains was –28.0‰, being closer to the measured value. The results indicate that isotope fractionation and temporal changes in carbon isotopic composition of atmospheric CO2 should be incorporated into the calculation for obtaining an estimate of carbon isotopic composition of the edible part of crops.