Display omitted •The substitutional Mn atom is preferable to diffuse along the 〈111〉 direction during the vacancy diffusion.•The Mn atom has a higher diffusion mobility than the Cr atom in the ferritic Fe-25%Cr Alloy.•The diffusion coefficient of Mn atom varies from 10−15 to 10−10 m2/s under the SOFC working temperature of 600 °C–800 °C. The doping manganese (Mn) in the ferritic Fe-Cr alloy could effectively improve its high temperature performance when applied as metallic interconnects of solid oxide fuel cells (SOFCs). A theoretical study of Mn diffusion property in such alloy is implemented. It is found that the substitutional Mn atom prefers to diffuse along the 〈111〉 crystal direction with activation energy of about 0.40 eV. The interstitial Mn atom diffuses across the crystal between two tetrahedral interstitials with activation energy of about 1.37 eV. Under the working temperature of SOFCs at 600℃-800℃, the magnitude of diffusion coefficients mainly varies from 10-15-10-10m2/s which is qualitatively consistent with the experimental result that diffusion rate following the order of DMn > DCr. Our results reveal the diffusion mechanism of Mn atom in the ferritic Fe-Cr alloy at the atomic scale, which provide theoretical guidance for further applications of metallic interconnects in SOFCs.