Crosslinked elastic fibers, made from a low density (0.875 g/cc) ethylene-octene copolymer, were studied after constrained at 300% elongation and annealed at different temperatures (40-80 degrees C) to simulate conditions encountered in yarn and textile processing. It is surprisingly found that the transition from pseudo hexagonal to orthorhombic structure is much faster under simultaneously constraining and annealing than that without strain. Almost a neat orthorhombic structure can be produced when the fiber is annealed at 60 degrees C. Annealing above 60 degrees C leads to mixed orthorhombic and pseudo-hexagonal structures. The average melting point increases with an increase in the fraction of orthorhombic phase. It is also surprisingly noted that the simultaneously constraining and annealing of the fiber can produce highly oriented crystals, even annealed at 80 degrees C (above the average melting point of 65 degrees C). The unique effect of annealing under large strain can be attributed to the crosslinking of the fiber, which makes it possible for the fiber to have strong chain orientation (even in molten state) under large strain. The strong chain orientation in melt leads to a faster structural transition from pseudo hexagonal to more stable orthorhombic structure. The strong chain orientation is also very likely the reason why highly oriented crystal and amorphous phases are formed, including the case where the fiber is annealed above melting point. These findings could be leveraged for improving thermal and mechanical properties of the fabrics made with such fibers. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3565-3573, 2013