Reaction of mesitylacetylene was carried out by annealing under high pressure (0.13 and 0.52 GPa). The products obtained were classified into soluble and insoluble products in chloroform. The insoluble product reacted under 0.13 GPa was the mesitylacetylene polymer. The soluble product reacted under 0.13 GPa was classified as the monomer and the oligomer [number-average molecular weight ((M) over bar(n)):390, weight-average molecular weight ((M) over bar(w)):453, Oligomer yield (O-y):36%]. The oligomer yield was accelerated by pressure [pressure: 0.13-0.52 GPa, (M) over bar(n):390-315, (M) over bar(w):453-968, O-y:36-98%]. Field desorption mass spectrum showed that the oligomer had cyclic structure. The result of the elementary analysis revealed that the insoluble product reacted under 0.52 GPa was a polycyclic aromatic compound.