Crosslinking behavior of internal acetylene units linked para to the aromatic rings was investigated by preparing polyimide from 4,4’-diaminodiphenylacetylene (p-intA) and 2,2’-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA). Polyimide was also prepared from 1,4-phenylenediamine (PDA) and 6FDA for comparison. The polymers were moderately to highly viscous at the stage of polyamide acid. Thermal imidization gave polyimide having acetylene units that are linked para to the aromatic connecting units. Differential scanning calorimetry (DSC) measurement of the polymer revealed that exotherm due to the crosslinking of the acetylene unit appeared at ca. 330 degrees C. After thermal treatment at high temperature such as 350 and 400 degrees C, onset of the exotherm shifted to higher temperature and the amount of the exotherm became smaller. The dynamic mechanical properties of the uncrosslinked polyimide film treated at 250 degrees C had a glass transition temperature (T-g) at 330 degrees C with a considerable drop in the storage modulus at this temperature. After the film was exposed to a higher temperature to induce crosslinking, the T-g was observed to increase to above 400 degrees C and the storage modulus was maintained to higher temperatures. Tensile properties of the polyimide showed that the films had good mechanical properties.