Single crystals of poly(dimethyl siloxane) were grown by cooling in an electron microscope. Selected area electron diffraction (SAED) patterns from the single crystals were categorized into four forms. According to the SAED patterns along with the X-ray diffraction pattern from the uniaxially oriented samples, structure models of these four types of crystal forms were proposed. In the four crystal forms, the molecular chains were thought to adopt almost the same conformation; the difference of the chain packing mode causes the crystal polymorphism. On the basis of the crystal structure models, the anomalous change of WAXD patterns with draw ratio, which is observed for the cross-linked samples under uniaxial stretching (M. Tosaka et al., Colloid Polym. Sci., 291 (2013) 2719.) was explained.