The iron boride layer growth kinetics in mild steel through the spark plasma sintering (SPS) pack-boriding technique is investigated at 850 °C with different boriding durations (maximum 240 min). Results show that both FeB and Fe2B layers form and grow on the mild steel surface with the FeB layer on the top of Fe2B sublayer in the samples with boriding duration less than 90 min. However, at longer boriding duration, the top FeB layer eventually ceases growing, starts to diminish, and, finally disappears completely by transforming into the Fe2B phase. Numerical simulation is implemented to explain this phenomenon. Subsequently, the diffusion coefficient of boron in FeB and Fe2B phase is obtained through fitting the experimental results into the model. The simulation results are found to be in good agreement with the experimental results, and the estimated diffusion coefficients of boron in FeB and Fe2B phases as 2.33 × 10-9 and 4.67 × 10-9 cm2/s, respectively. Both the simulation and experimental results reveal that the Fe2B mono-phase layer can be obtained through the transformation of FeB to Fe2B phase due to the depletion of boron concentration in the boriding medium, and is indifferent to the formation of FeB phase at the very onset of the boriding process. This provides a new approach to overcome the side effect of FeB formation in borided components. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.