The dynamics of poly(dimethylsilane) (PDMS) in the mesophase was investigated by DSC and Si-29 solid-state NMR. A solid-solid transition is observed at 166 degrees C based on DSC measurements. Below the transition temperature, typical chemical shift anisotropy (CSA) spectra are obtained by the Si-29 solid-state NMR with cross-polarization (CP) preparation for the static PDMS sample. In contrast, the intensities at the isotropic chemical shift are suppressed for the CSA spectra above the transition temperature. The intensity attenuation, termed "magic angle hole", in the CSA spectra is theoretically derived for uniaxially rotating solids. Therefore, it was found that PDMS undergoes a uniaxial rotation motion above the transition temperature. Theoretically, both the direct polarization (DP) experiments without the CP process and ultraslow magic angle spinning (MAS) experiments retrieve standard CSA line shapes. Our DP and ultraslow MAS experiments confirm the theoretical consideration. The retrieved CSA spectra have axially symmetric line shapes, which also confirm the above uniaxial dynamics. On the basis of the solid-state NMR, we clearly show that the solid-solid transition at 166 degrees C is a rigid monoclinic-mobile rotator phase transition.