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Due to its great performance, tungsten (W) and W-based alloys have been considered as the candidates for plasma-faced materials in future fusion reactors. However, the radiation-induced segregation under the reactor environment is one of the key issues to limit the long-term service properties of W-based alloys, and the understanding of nucleation and growth mechanisms of Re-rich precipitates is still incomplete. In the present study, both molecular dynamics (MD) and variance-constrained semi-grand canonical ensemble Monte Carlo (VC-SGC-MC) methods are employed to investigate the nucleation and growth mechanisms of Re clusters precipitated in W-Re alloys under cumulative flux irradiation. It is found that all segregated Re-rich clusters in the model alloys without irradiation are a plate-like structure (χ-phase) which orientates along the <110>, while they change to a needle-like structure (σ-phase …