A series of organo-soluble aromatic polyimides have been synthesized from four different dianhydrides and two different diamines. The main purpose of this research was to develop new toughening agents from thermoplastic polyimides for epoxy resins. Thus, the solubility of these polyimides in epoxy resins is crucial to obtain a homogeneous mixture before curing. The structure dependence of these polyimides on solubility, thermal and mechanical properties was investigated in this study. The subglass relaxation processes in polyimides were also discussed based on the non-cooperative and cooperative motion proposed by Starkweather. A suitable polyimide has been found for toughening epoxy resins.