Abstract: Lower temperature irradiation embrittlement is one primary issue limiting the use of ferritic/martensitic steels. The primary mechanism of lower temperature irradiation embrittlement is the obstruction of dislocation motion by nano-metric defects, such as dislocation loop and α′ precipitate (Cr clusters), induced by irradiation. Using molecular dynamics and Metropolis Monte Carlo (MMC), the mechanism of Cr precipitate and segregation on dislocation loop in FeCr alloys (the models of F/M steels) was studied. On this basis, the dependence of Cr clusters on Cr contents, and the impacts of loop size, loop type and Cr contents in FeCr alloys on the Cr enrichment on loop were explored. The results show that after simulation, Cr clusters were formed in FeCr alloys with higher Cr contents (>9%) and the size of Cr clusters increases with Cr contents in FeCr alloy. For all the studied FeCr alloys here, under the interaction of tensile stress fields, Cr elements can segregate on the outside of dislocation loop after thermodynamics simulation, and enrichment level of Cr on loop increases with the Cr contents in FeCr alloys. For lower Cr contents, the effect of Cr on hardening only needs to consider the Cr segregation on loops, while for higher Cr contents, the influence of Cr contents on the irradiation hardening needs the synthetical study of Cr clusters and Cr segregation on loops.