A diimide-diacid (DIDA) curing agent of N,N＇-(4,4＇-diphenylsulfone) bis(trimellitimide) was synthesized. Through the spectral characterization of infrared (i.r.), H-1 n.m.r., C-13 n.m.r., and of elemental analysis (EA), the DIDA structure was confirmed. The curing behaviour and resultant thermal properties of epoxy resin (Epon 828) with DIDA were studied by differential scanning calorimetry (d.s.c.) and thermogravimetric analysis (t.g.a), respectively. In order to improve the processing ability, DIDA was partially replaced by phthalic anhydride (PA) in the formulations carried. The thermal resistance depended on the relative proportions of curing agents in the formulations.