A new high-performance thermoplastic polyimide has recently been synthesized from 3,3’,4,4’-benzophenonecarboxylic dianhydride (BTDA) and 2,2’-dimethyl-1,3-(4-aminophenoxy)propane (DMDA). It has been shown to possess good dimensional and thermal stability, as well as exhibiting excellent mechanical properties, in particular the extraordinary toughness of BTDA-DMDA with a fracture energy G(1C) value of 4.28 kJ m(-2), which exceeds that of other high-performance materials by about 100%. Clearly, this material has great potential as a matrix resin for high-temperature, high-performance composites. This report focuses on the study of this polymer’s structure, crystal morphology, crystal-amorphous interface and relaxation processes in order to find the origin of this unusually high toughness. In addition to this, we have also examined the matrix/fiber interfaces of BTDA-DMDA-based carbon fiber and polyimide fiber-reinforced composites via their mechanical properties, and then their crystallization and relaxation behaviors.