The freezing behaviors of Al nanowires with different section sizes and cooling rates are studied by using the classic molecular dynamics simulation via embedded atom potentials. In order to invesligate the evolution of the local clusters in the transformation of Al nanowires, the pair analysis technique is employed. The simulation results indicate that the final structure of Al nanowires is strongly affected not only by cooling rate, but also by the size effect during solidification from liquid. At a rapid cooling rate, the final structures are all helical multi-shelled structures. However, at a slower cooling rate, the structure changes from helical multi-shelled to crystalline via near-hexagonal shell structure with the increase of section size except that the thinnest Al nanowires break down.