The structure of a side chain liquid crystalline poly(silylenemethylene) (-(SiCH3R-CH2)-: R = O(CH2)(11)O-Ph-Ph-CN, Ph = phenyl) (CN-11) has been studied by X-ray diffraction and differential scanning calorimetry (DSC). The DSC results showed that CN-11 has transitions at similar to92 degreesC (T-2) and similar to147 degreesC (T-1) during both cooling and immediate heating. A third transition occurred at similar to50 degreesC (T-3) during heating after annealing at room temperature. The X-ray fiber pattern of the CN-11 annealed at room temperature showed several wide and small angle reflections which were indexed by a monoclinic unit cell with parameters a = 16.8 Angstrom, b = 7.42 Angstrom, c = 43.6 Angstrom and beta = 102.1degrees (b: fiber direction), representing a crystal structure with layer thickness of similar to43 Angstrom. Upon heating at T-3, the crystal structure became less ordered (but somewhat more ordered than smectic Angstrom (S-A) and smectic C (S-C)). This was followed by S-A (or S-C) phase at T-2, and ultimately an isotropic state (I) at T-1. The observed layer thickness (similar to43 Angstrom) is about similar to1.5 times the most extended side chain length, indicating a double-layer structure with tilted or interdigitated side chains. The X-ray fiber pattern had a four-point pattern at d = 4.52 Angstrom, suggesting that the side chains in the crystal are likely to be tilted by 56degrees from the polymer fiber axis.