The natural draw ratio of metallocen catalyzed high density polyethylenes was investigated with different crosshead speeds, molecular weights, and the cross-section shapes of sample specimens where the elastic components included in the conventional natural draw ratio such as residual strain and elastic aftereffect were eliminated. The perfect plastic deformation took place below a critical crosshead speed, whereas void formation occurred above the critical one. The natural draw ratio without elastic components and void formation was found to be dependent on the molecular weight of samples and the dimension of their specimens. On the basis of the SEM images, we proposed a simple structural model for explaining the necking formation in addition to the molecular weight and the cross-sectional shape dependences of the natural draw ratio. (C) 2011 Elsevier Ltd. All rights reserved.