Low ductility metals are known to have ductility improved by composing with high ductility metals in layers, for example, cladding metals. The purpose of this investigation is to clarify the formation and growth mechanism of necking ( thinning ) and the effect of the necks on mechanism of ductility enhancement in such metals. Simple tension tests are performed on aluminum sheets which are locally annealed by using a high frequency induction heating device and have hardness distribution in width direction. A geometrical model for the necking pattern is proposed, and the effects of annealed width on limit elongation and on the angle, pitch and pattern of necking are discussed in detail. Comparisons are made between the center and the edge annealed sheets. It is clearly demonstrated that the limit elongation and the ultimate strength of such composite metals can be assessed by applying Swift's instability criterion ( maximum load condition ) to the total load.