The growth rate equation of Fe-Zn alloy layer was represented by x = Kt, and hence the growth of alloy layer was considered to be controlled by diffusion process. The constituent of alloy layer formed on the steel surface was identified to be intermetallic compound of Fe3Zn10 and FeZn10. The ultimate tensile strength and elongation of galvanized steel showed a nearly constant value at the thickness below about 30$mu extrm{m}$, and both properties decreased with increasing thickness above about 30$mu extrm{m}$. In the case of galvanied steel with a great thickness of alloy layer, crack was formed below yield point of base metal, which is considered to be attributed to the alloy layer failure.