Fiber formation from highly stretched melt and mechanical deformation of the fiber are investigated by molecular dynamics simulations. Polyethylene molecules of moderate chain length comprising 513 united atoms are simulated under constant temperature and stress (NT iota) conditions. Rapid crystallization into fiber structures of alternating crystalline amorphous layers is observed, where emergence of very thin lamellae and their subsequent thickening are found to be the basic mechanism of the fiber formation. The incipient lamellae are also suggested to have an intermediate degree of order. Mechanical elongation of the fiber along its axis reveals initial elastic deformation and large reorientation of the crystalline chains, which is followed by specific yielding due to chain slips in the crystals. Further elongation of the fiber leads to the formation of crazes and microfibrils. On the other hand, the transverse deformation of the oriented sample exhibits much lower Young's modulus and larger plastic deformation. During the transverse deformation, the fiber texture is severely distorted giving rise to the pronounced melting and recrystallization (or breaking and reformation) of the crystalline textures, through which the oriented fiber accomplishes nearly complete 90 degrees reorientation. (c) 2013 Elsevier Ltd. All rights reserved.