Abstract: Lower temperature irradiation embrittlement is one primary issue limiting the use of reduced activation ferritic/martensitic (RAFM) steels. The primary mechanism of lower temperature irradiation embrittlement is the obstruction of dislocation motion by nano-metric defects, such as dislocation loop and precipitation, induced by irradiation. Using molecular dynamics, the interaction mechanism between edge dislocation line and 1/2〈111〉 interstitial dislocation loop in bcc-Fe was studied, and the effect of Cr segregation at interstitial dislocation loop on irradiation hardening was analyzed. The results show that the critical resolved shearing strain (CRSS) for dislocation line to break away from loop is related with the Burgers vector (BV) of loop. Under the condition studied here, within a certain temperature range, Cr segregation at loop increases the CRSS for dislocation line to break away, thus enhances hardening.