Reaction of diphenyldiacetylene (1,4-diphenylbutadiyne) by annealing under elevate pressure (0.1-500 MPa) was carried out. Diphenyldiacetylene reacted at 210-degrees-C with appearance of gas, and this temperature was independent of the pressure. The measurement of high pressure differential thermal analysis (DTA) revealed that the reaction temperature under elevated pressure was below 24-42-degrees-C of the exothermic peak temperature. This implied that exothermic reaction occurred under elevated pressure. Elementary analysis, gel permeation chromatography (GPC), FTIR, Raman scattering, and high resolution C-13-NMR experiments were performed to characterize the structure of the product. It was indicated that the product was a mixture of derivative of condensed polycyclic aromatic compound with phenyl group and diphenyldiacetylene oligomer. The fraction of the derivatives increased with increasing pressure, and pressure accelerated the dehydrogenation of the derivatives. The number-averaged molecular weight (M(n)) of the diphenyldiacetylene oligomer was 470-610 and the weight-averaged molecular weight (M(w)) was 1700-2300. It was considered that the oligomer had a polyacene-based structure.