The solidification structure and hardness distribution in aluminum alloy duplex pipes produced by a two-step centrifugal casting have been investigated. In this process, two kinds of molten metals, i.e., the first melt and the second melt with a higher liquidus temperature were cast in sequence at a given interval into a rotating mold of a centrifugal caster. An Al-12mass%Si alloy was used for the first melt, and an Al-30mass%Ni or Al-32mass%Si-0.1mass%P alloy was used for the second melt. The second melt was cast after the solidified shell of the first melt had formed. The resultant cast pipes consisted of an outer side layer and a composite layer containing fine primary crystals. The outer side layer was a portion of the solidified shell of the first melt that survived after the contact with the higher-temperature second melt. The composite layer consisted of one or two layer(s). When the volume of the remelted part of the solidified shell was large, all the second melt mixed into the first melt and the resulted mixed melt formed the composite layer. On the other hand, the composite layer formed only from the second melt when the temperature of the solidified shell was low. In the intermediate case, the composite layer consisted of these two types of the layers.