The composition, microstructure and formation mechanism of the bottom dross in hot-dipping Zn-0.1%Ni alloy bath have been investigated through XRF, XRD and SEM. It is found that the bottom dross is an aggregation consisting of FeZn13(ζ), FeZn15and a small amount of Ni3Zn22. The formation mechanism of bottom dross can be described that loose ζ phase formed on the steel substrate and then flakes off from substrate; ferroalloy is formed and precipitated from the melt with diffusion of Fe into molten zinc if the dissolved Fe surpass its solubility in the molten zinc. It also indicated that there exists a trace of Ni in the bottom dross in hot-dipping Zn-0.1%Ni alloy bath.