Three series of polydiphenylsiloxane (PDPS)-containing diblock copolymers were synthesized with controlled molecular weight and composition by sequential addition of the corresponding comonomers through a living anionic polymerization. These copolymers included poly(diphenylsiloxane-b-dimethylsiloxane) (PDPS/PDMS), poly(diphenylsiloxane-b-methylphenylsiloxane) (PDPS/PMPS), and poly(diphenylsiloxane-b-methyltrifluoropropylsiloxane) (PDPS/PMFPS). The structural effect of the soft segment of these PDPS-containing diblock coploymers on the mesophase transition behaviors was examined. The observations with differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were combined to determine the actual thermal transition behaviors. Critical compositions for the appearance of these thermal transitions were determined for each series of the diblock copolymers. The thermal transition properties, such as temperature and peak shape were also correlated to the structures of the copolymers.