Carrier transport properties in poly(methylphenylsilane) films were studied with interest focusing on the variation of Bässler's disorder parameters with changing molecular weights. There was no apparent correlation between the charge transport properties and the molecular density and free volume of the samples, the latter being measured by a positron annihilation technique. This result led us to a model explaining the variation in Σ with changing molecular weights, a model which is based on a hypothesis that ca. 10 silylene units at the end of a polymer chain do not participate in forming a σ-conjugated domain. The validity of this model was demonstrated by UV absorption measurements. The variation of μ₀ was interpreted in terms of the partial orientation of the main chains induced by the mechanical force applied by a bar-coater. It is discussed that μ₀ is sensitive to even the slightest orientation of the main chains while the other disorder parameters are not.