The electroplated steel sheets were heated during the short periods(10~60 seconds) at high temperature ($360^{circ}C$～$500^{circ}C$) in order to investigate thermal diffusion behaviors. When the steel sheets were heated for 10 seconds, all the coated layers were alloyed at $420^{circ}C$ but at temperature lower than $400^{circ}C$ the $eta$ phase partially remained on the coated surface. At higher temperature, the longer the time for heat treatment the iron contents were increased in coated layer but the glossiness and whiteness of the coated surface were decreased. While the alloying phases of $eta$, $zeta$, $delta_1$ and $Gamma$ were appeared in the coated layer at the heat treatment temperature of $360^{circ}C$, the phase was disappeared at $420^{circ}C$ but the rests grew in size at the temperature of $440^{circ}C$. When the heat treatment temperature and heating time were increased, the thickness of $Gamma$ phase was rapidly increased to 0.8 $mu extrm{m}$. The optimum conditions for the heat treatment to prevent powdering of coated layer were obtained to heat it for 30 seconds at $400^{circ}C$ and 10 seconds at $440^{circ}C$, and the iron content in coated layer was suited to be 10 percents.