A series of poly(dimethylsiloxane-co-diphenylsiloxane)s with different molar content of diphenylsiloxane unit (from 0.91 to 9.07 mol %) were synthesized and characterized by use of H-1 NMR, gel permeation chromatography (GPC), the differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD). The presence of diphenylsiloxane unit results in a decrease in the "orderness" of the crystal phase and aggrandizement in degree of lattice distortion. By calculating the sequence length of dimethylsiloxane unit of the random copolymer, it can be concluded that the maximum average sequence length of dimethylsiloxane units required for the copolymer to be amorphous is 11, and increasing the dimethylsiloxane sequence length will favour crystallization. The reduction in sequence length of dimethylsiloxane unit leads to that the three-dimensional crystal shape is destroyed and crystal shape transforms into two-dimensional or one-dimensional, and finally disappeared. (C) 2007 Wiley Periodicals, Inc.