Polypyromellitimides show excellent thermal stability due to their rigid aromatic chain molecules. In the present paper, the viscoelastic properties of such polymers are correlated to their rigid chain structure. The viscoelastic properties of polypyromellitimides prepared with various degrees of cyclization from polypyromellitamic acid film are measured with a tensile oscillating apparatus at 5Hz in the range of -60∼90°C. And the viscoelastic properties of polypyromellitimide, Kapton H film, are also measured at a higher temperature range of 20∼500°C by means of a torsional pendulum. Three mechanical ralaxations were found at -60°C and 70°C by tensile measurement, and at 60°C and 330°C by torsional measurement. The lowest one is ascribed to the absorbed water as previously pointed out. The middle one is found also to be related to the absorbed water. The highest one, which is revealed in this work for the first time, is assigned to rearrangement of the oriented molecules at higher temperatures.