Plastic instability in uniaxial tension of commercial purity Al has been studied with the emphasis of effects of thickness in cold worked specimens and recrystallized specimens. The thickness change gave rise to change in stress state and the amount of strain localization in specimen after diffuse necking. Therefore the thickness of speci- men could control modes of plastic instability. Regardless of recrystallized or cold worked state, the necking mode changed from diffuse necking to local necking, at about 1.5 .approx. 2 mm in thickness.