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 D-1 of the B-monovacancy complex mechanism was calculated without considering the backward jump of the B atoms. The calculated D-1 = 1.26 x 10(-4) x exp((-)2.01eV/k(B)T) m(2).s(-1) is consistent with the reported results from experiments.