Diphenylsiloxane oligomers (MN = 1200-4200, MW/MN < 1.3) without dimethylsiloxy end-group have been prepared and characterized. Cooling of the oligomer with a MN of 2200 or higher from the isotropic state results in the formation of the birefringent mesophase. A spherulite structure is observed upon further cooling from the mesophase. The increase of the mesophase temperature interval on the increasing molecular weight reflects that the increase of the molecular weight will improve the stability of the mesophase. Multiple melting transitions are observed from the spherulite to the mesophase for the oligomers with MN < 3800 in the d.s.c. hearing scan. The end-group effect on the phase behaviours of diphenylsiloxane oligomers was revealed by comparing the phase diagram of three diphenylsiloxane oligomer; with varied flexible end-groups. The phase diagram for these oligomers shows a flexible end-group effect on lowering the melting and isotropization temperature. With the increasing flexible chain-length, the critical molecular weight for mesophase formation is increased. The flexible end-group also has an effect on decreasing the endothermic heat of melting measured by d.s.c. In addition, a free-energy diagram based on thermodynamics is proposed to explain the absence of mesophase of the oligomers with MN lower than 1900. The mesophase is suggested as lamellar crystals which behave both in bright needles and plates surrounded by the isotropic melts.