The mechanical properties and the phase behaviour of poly(dimethylsilylene-co-methyl-n-propylsilylene) (PDM-co-MPS), were investigated by the measurements of dynamic viscoelasticity, wide angle X-ray diffraction, thermal analysis and UV spectrum. PDM-co-MPS was soluble in common organic solvents if the ratio of dimethylsilylene (DM) unit to methyl-n-propylsilylene (MP) unit, DM/MP, was lower than 70/30. The crystal phase was transformed into the columnar mesophase above 80-90 degrees C, and the disordering of conformation and the orientation relaxation proceeded with further rise in temperature in the range of 80-130 degrees C. The PDM-co-MPS films were extensible only in the narrow temperature range, in which the crystal to mesophase transition started to occur, because the structural relaxation in the columnar mesophase reduced the extensibility of the PDM-co-MPS films. The oriented films of PDM-co-MPS were prepared by stretching the solution-cast films at 70-80 degrees C to a draw ratio of 7-8. The elastic modulus was much improved in the hot drawn film relative to that of the isotropic films, and the increase of modulus was related to the high degree of orientation in the crystal phase. A large loss tan delta was observed at -20 to -10 degrees C, and the dynamic storage modulus markedly decreased with the rise in temperature in the range of this transition. As the low temperature transition was not detected in the DSC and spectroscopic measurements, it would be assigned to the local molecular motion of the n-propyl side group and/or to the glass transition.