A micromechanics model was developed to simulate creep fracture of ceramics at high temperatures and material properties pertinent to zirconium diboride (ZrB2) were adopted in the simulation. Creep fracture is a process of nucleation, growth, and coalescence of cavities along the grain boundaries in a localized and inhomogeneous manner. Based on the grain boundary cavitation process, creep fracture can be categorized into cavity nucleation-controlled and cavity growth-controlled processes. On the other hand, based on the deformation mechanism, the separation between two adjacent grain boundaries can be categorized into diffusion-controlled and creep-controlled mechanisms. In this study, a parametric study was performed to examine the effects of applied stress, cavity nucleation parameter, grain boundary diffusivity, and applied strain rate on cavity nucleation-controlled versus growth-controlled process as well as diffusion-controlled vs. creep-controlled mechanism during creep fracture of ZrB2.