Abstract: Tritium is the most important fuel in fusion reactors and one of the important radionuclides in fission reactors. The study of the diffusion behavior of tritium in graphite is of great significance to understand and grasp the transport process of tritium in nuclear energy systems. Regarding the measurement methods of diffusion coefficients, they can be classified into three types: permeation method, profiling method and release method. There are few experiments to measure the diffusion coefficient of tritium in graphite, and the obtained data vary greatly. Using the irradiated graphite spheres in 10 MW High Temperature Gas-cooled Experimental Reactor (HTR-10), two methods were proposed to measure the diffusion coefficient of tritium in graphite within a certain range of temperature. One is based on the cumulative release fraction of tritium from graphite, and the other is based on the release rate of tritium from graphite. The quantitative relationships between the cumulative release fraction and diffusion coefficient, and between the release rate and diffusion coefficient in spherical materials were derived in detail, and the factors that may have an influence on the experimental measurement results were discussed. The corresponding experimental systems were designed and the characteristics of the two measurement methods were compared. The experimental method based on the cumulative release fraction is suitable for the measurement of materials containing a variety of easily released nuclides, such as noble gas nuclides or ¹⁴C. The cumulative release fraction of tritium can be measured, and the measurement results will be not affected by the presence of other radionuclides. The advantage of the experimental method based on the release rate is that it is a fast and real-time measurement method. However, if other radionuclides are present in the irradiated graphite spheres, they can interfere with the online measured values of tritium. The factors affecting the diffusion behavior of tritium in irradiated graphite spheres were discussed in conjunction with the results of the tritium source term study on HTR-10. The diffusion behavior of tritium in HTR-10 irradiated graphite spheres may be affected by the microstructure and composition of the material. Future studies based on the first-principle calculations should consider the impurities, defects, and grain boundaries of graphite materials to determine the theoretical diffusion coefficients, and compare them with experimental measurements to elucidate the diffusion mechanism of tritium in graphite materials. Current study can provide an important research basis for determining the diffusion coefficient of tritium in irradiated graphite spheres of HTR-10 and mastering the transport mechanism of tritium in graphite materials in the future.