The relaxation behaviors of poly(dimethylsiloxane-co-diphenylsiloxane)s with different compositions were investigated using dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). It is indicated that the content of Ph2SiO unit, which is closely associated with crystallinity of polysiloxane, has a remarkable influence on its relaxation behavior. Two-phase (crystalline and amorphous phase) structure in the semicrystalline polysiloxane of the present system can be determined for discussing relaxation behavior. An increase in relaxation strength can be reasoned to a cooperative effect of decrease in fraction of crystalline phase and increase in friction between molecular chains. And enhancements in glass transition temperature (T-g) and effective activation energy for glass transition (E-a(eff)) were ascribed more to the stiffness imposed by Ph2SiO unit than decrease in fraction of crystalline phase. (C) 2008 Wiley Periodicals, Inc.