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The embrittlement of nickel-based structural alloys by fission-produced tellurium (Te) is a major chal-lenge for molten salt reactors (MSR). In this study, the effects of thermal exposure time on tellurium diffusion in a candidate MSR structural alloy (Ni–16Mo–7Cr–4Fe) and the consequent mechanical property degradation of the alloy were investigated through surrogate diffusion exper-iments at 700 ℃. The results show that some tellurium reacted with the alloy to form tellurides on the surface, while some tellurium diffused into the alloy along grain boundaries. Ni3Te2 and CrTe were the most stable reaction products at the tested temperature, and the formation of CrTe on the surface induced the Cr depletion at grain boundaries of the alloy. The diffusion depth of Te increased gradually with thermal exposure time, and the diffusion rate kept stable within the test duration of up to 3000 h. The Te diffusion in the alloy caused the embrit-tlement of grain boundaries, inducing crack formation and strength degradation in tensile test at room temperature.