The crystalline structure of poly(decamethylene sebacate) has been studied using transmission electron microscopy and X-ray diffraction. The polymer shows polymorphism, since fibers corresponding to both monoclinic and orthorhombic unit cells can be obtained depending on the draw and annealing conditions. The second structure can also be obtained from solution crystallization in polar solvents such as 1-hexanol. Molecular packing of this structure has been studied in detail taking into account both diffraction data and energy calculations. The unit cell contains two chain segments with a slightly distorted all-trans conformation and a chain setting angle similar to that reported for polyethylene. The two chain segments are shifted along the chain axis direction, as they are related by a glide plane n. Lamellar crystals are sectorized and show a regular folding surface structure. Two kinds of folds, constituted by methylene or by ester groups, can be deduced from the space group symmetry and the surface decoration.