Fe-Zn reaction was investigated by annealing a galvanized 22MnB5 steel sheet at 500–900 ° C for 5 min. The observed Fe-Zn intermetallic compounds within the coating depended on the annealing temperature. As the temperature increased, the coating phase changed from δ via Γ1 to Zn-rich ferrite. At the temperature lower than or equal to 700 ° C, Zn seems to have diffused along the grain boundaries of substrate, which formed fine Zn-rich ferrite grains. Annealing at higher temperature produced coarse Zn-rich ferrite grains which contributed to significant thickening of the coating layer. By tensile straining at the annealing temperature, it was found that the fine Zn-rich ferrite along the grain boundaries was the origin of the microcracks in the substrate.