The diffusion mechanism of boron in fcc-Fe was studied by first-principles calculations. The sites where B atoms tend to occupy and the diffusion behavior were calculated. Results indicated that the main mechanism of boron diffusion in fcc-Fe was the B–monovacancy complex mechanism instead of the interstitial mechanism. The diffusion coefficient D1 of the B–monovacancy complex mechanism was calculated without considering the backward jump of the B atoms. The calculated D1=1.26×10−4×exp(–2.01eV/kBT) m2·s−1 is consistent with the reported results from experiments.