Poly(tetramethyl-p-silphenylenesiloxane) (PTMPS) containing phenyl rings in the main chain is crystallized from solution and melt. Differential scanning calorimetry (DSC) measurements gave a single endotherm for a solution-crystallized sample but double endotherms for melt-crystallized film, both of which occurred at temperatures of 120-135 degrees C. A combination of in situ X-ray and NMR measurements during heating of these PTMPS samples indicates that the sliding of phenyl rings, which are diagonally packed within the crystals, begins before melting endotherm region. For the solution-crystallized sample, this molecular sliding induces characteristic lamellar doubling phenomena, leaving a single higher-temperature-side endotherm corresponding to melting of the resultant thickened lamellae. In contrast, such lamellar thickening is restricted for the melt-crystallized sample due to its entangled amorphous state. NMR measurement results indicate that the molecular motion of the phenyl rings exhibits remarkable anisotropy even in the molten state, suggesting rigidity of the PTMPS molecules.