In-situ structural changes at the lamellar level during uniaxial deformation and subsequent relaxation of a semicrystalline ethylene-propylene (EP) copolymer containing 78 wt % ethylene were studied by time-resolved synchrotron small-angle X-ray scattering (SAXS) at room temperature (25 degreesC). During the initial stage of deformation, the long period was found to increase in the stretching direction and decrease in the transverse direction. In addition to the elastic change of the lamellar structure, further analysis suggested that there was an additional contribution of strain-induced "melting", which began at a relatively low strain (about 10%) and was directionally dependent. At 75% strain, new crystals, possibly with extended-chain conformation and needlelike microvoids, started to form, which coexisted with the original crystals (dominated by folded-chain conformation). Because of the destruction of original crystals, the applied stress was found to decrease slightly after the yield point. Above 120% strain, the stress increased linearly with strain partially due to the formation of new crystals. Long periods of new crystals decreased with strain upon further stretching. During relaxation, a large amount of well-oriented new crystals remained in the sample. As a result, only 55% of the original sample length was recovered. The long period of the new crystals after relaxation was about 15 nm, much smaller than that of the original crystals (24 nm). Strain-induced microvoids were also found to remain in the sample after relaxation.