Xenon oscillations of large graphite-moderated reactors have been analyzed by a multi-group diffusion code with two-and three-dimensional core models to study the effects of the geometric core models and the neutron energy group structures on the evaluation of the Xe oscillation behavior.The study clarified the following. It is important for accurate Xe oscillation simulations to use the neutron energy group structure that describes well the large change in the absorption cross section of Xe in the thermal energy range of 0.1~0.65 eV, because the energy structure in this energy range has significant influences on the amplitude and the period of oscillations in power distributions. Two-dimensional R-Z models can be used instead of three-dimensional R-0-Z models for evaluation of the threshold power of Xe oscillation, but two-dimensional R-0 models cannot be used for evaluation of the threshold power. Although the threshold power evaluated with the R-0-Z models coincides with that of the R-Z models, it does not coincide with that of the R-0 models.