Abstract: In order to study the differences of HTO uptake and OBT formation in peanut following a short atmospheric HTO release in nighttime and daytime condition, the potted peanut plants were exposed to evaporated HTO for 1 h in an exposure box at the flowering and podding stages under nighttime and daytime, respectively. The concentrations of tissue free water tritium (TFWT) and organically bound tritium (OBT) in leaves and peanut kernels were measured at the end of the exposure and at the harvest, respectively. The results show that HTO uptake in peanut leaves during the nighttime is several times lower than that during the daytime. At the flowering stage, the relative TFWT and OBT concentrations in leaves under night conditions at the end of the exposure are about one sixth and one third from the concentration for day conditions. And at the harvest, the TFWT and OBT concentrations of leaves are lower than that at the end of exposure. At the podding stage, the relative TFWT and OBT concentrations in leaves and peanut kernels under night conditions are one third, two third and two third, 1.13 times from the concentration under day conditions at the end of exposure. At the harvest, the TFWT concentration in peanut kernels is higher than that at end of the exposure, and the relative OBT concentration in leaves increases by factor of 1.7, but the OBT concentration in peanut kernels is only one fourth of that at the end of exposure under night condition. At the harvest, the OBT concentration in leaves is lower than that at the end of exposure, however the OBT concentration in kernels increases under day condition. Following a short atmospheric HTO release in nighttime and daytime conditions at the different growth and development stages, the variation tendency of TFWT and OBT in leaves and kernels are different. Therefore, the parameters of tritium concentration prediction model in plant should base the growth stages for accidental release of atmospheric HTO, which could make the ingestion doses for tritium be estimated accurately and objectively.