Molecular dynamics calculations are performed to study the structures of armchair carbon nanotubes (CNTs) encapsulating Ni nanowires and their helicities and thermal stabilities are discussed. It is found that Ni nanowires are of multiple helical tubular structure and each layer is composed of several Ni atom chains. Different layers of Ni nanowires have different helicities and the helicities of Ni nanowires in the inner layer are greater than those of the outer one. What is more, the helicity will be greatest when the number of the Ni atom chains is an integer multiple of three. As the increases of the diameters of the CNTs, the helicities of Ni nanowires change periodically. The structure and helicity only have tiny variations even at high temperature as the CNTs can protect the Ni nanowire.