Modification of steel bolt coating layers was studied using two step Hot-Dipping. The sample was at first dipped into primary Zn melt, and successively into secondary Zn-7mass%Al melt. To clean the steel surface, and prevent it from oxidation, two fluxes, aqueous solution and molten salt, were used before the first dip. The formation and morphology of the coating layer were studied using SEM and EPMA. When the aqueous solution flux was used, an alloy layer consists of a palisade δ1 phase and granular (δ1
+η) phases, and an ζ phase was formed on the steel surface after the dipping into primary Zn melt. In the sample pretreated with molten salt flux, δ1
phase grew thickly from the steel surface, and ζ phase grew successively, and minimal (δ1
+η) phases were ovserved. The alloy layer thickness formed after dipping into secondary melt was controlled by the morphology of the alloy layer formed at the first dipping, i. e., if the alloy layer includes η phase, Fe4
phase, formed by the reaction with Zn-Al melt, is dispersed, resulting in spalling and dislodgment of the alloy layer during centrifugation. When the molten salt flux was used, uniform coating layer thickness more than 50μm was obtained in two step Hot-Dipping.